NOVELTY - Preparation of solar polysilicon comprises using highly pure silicon stone and total petroleum coke carbon reductant as material; preparing compound additive; fully mixing compound additive, reductant and silicon; performing thermal reduction at 2000-2400 degrees C in ore heat furnace; performing furnace external oxygen blowing refining and slag cleaning for melt silicon liquid; performing slag cleaning oxidation reaction for melt special industrial silicon; melting compound slag agent in induction furnace; and pouring melt special industrial silicon in slag solvent pool.    USE - Preparation of solar polysilicon used for parallel-grid power station.    ADVANTAGE - The method produces solar polysilicon with boron content of less than or equal to 0.245 ppmw and phosphorus content of less than or equal to 0.43 ppmw, and purity of greater than or equal to 99.9999%. The method has short manufacturing technique flow, low electricity consumption, less environment pollution, and high product purity.    DETAILED DESCRIPTION - Preparation of solar polysilicon comprises using highly pure silicon stone and total petroleum coke carbon reductant as material; preparing compound additive comprising (%) calcium fluoride (65), calcium oxide (25), and sodium silicate (10); fully mixing compound additive, reductant and silicon at weight ratio of 1:(3-5):(8-12); performing thermal reduction at 2000-2400 degrees C in ore heat furnace to directly reduce element silicon in silicon oxide into simple substance silicon; performing furnace external oxygen blowing refining and slag cleaning for melt silicon liquid to obtain melt special industrial silicon; performing slag cleaning oxidation reaction for melt special industrial silicon to remove boron; melting compound slag agent in induction furnace to form slag agent melt pool; mixing two slag agents according to weight ratio of compound slag agent sodium silicate, silicon dioxide, and calcium oxide at (5.5-7):(2.5-3):(0.5-1), in which mass ratio of slag agent to special industrial silicon is 1:(1-2.5) and slag silicon ratio is 1-0.4; pouring melt special industrial silicon in slag solvent pool; performing slag cleaning oxidation between silicon liquid and slag agent in induction furnace while electromagnetic stirring; keeping temperature at 1700-1800 degrees C; performing slag cleaning for 2 times each at 30 minutes to reduce content of impurities comprising boron, aluminum, and calcium in silicon liquid; separating slag liquid through siphon principle by refining industrial silicon; pouring silicon liquid in directional condenser; standing and insulating; condensing for 48-72 hours; taking out after cooling silicon ingot; grinding four sides of silicon ingot and bottom surface; cutting off upper part metal impurity enrichment region (i.e. 10-15% of total height of silicon ingot) to obtain refined industrial silicon with boron (B) content of less than or equal to 0.3 ppmw and low metal impurity; crushing refined industrial silicon to 100-200 meshes to perform wet method metallurgy; performing acid leaching processing using (%) hydrochloric acid (5-10), hydrofluoric acid (5-10), aqua regia with hydrochloric acid concentration of (36-38) and nitric acid concentration of (65-68), hydrofluoric acid (20-30), and hydrogen peroxide (1-3), in which solid to liquid ratio is 1:3 and acid leaching processing time is 16 hours; stirring for 1-5 minutes once every year; cleaning and rinsing using deionized water; drying through centrifuging; cleaning for several times using deionized water until neutral to obtain highly pure silicon powder with boron content of less than or equal to 0.25 ppmw; centrifuging; drying and packing; performing high efficiency depth directional condensation to remove impurity using polysilicon ingot casting furnace; melting highly pure silicon powder at 1440-1550 degrees C; volatilizing for 2-3 hours at 5 kPa vacuum; condensing at 0.15-0.28 mm/minute; condensing and crystallizing; cooling ingot and discharging; grinding silicon ingot four sides and bottom surface; cutting off upper part metal impurity enrichment region which is 12-18% of total height of silicon ingot to obtain 5.5-6 N level directional polysilicon; condensing; performing electron beam bombardment purification for 5.5-6N polysilicon at 10-2 to 10-4 kPa vacuum and 1800 degrees C; and removing phosphorus impurity to obtain high purity 6-7 N solar polysilicon with low boron and low phosphorus contents.