NOVELTY - Preparing wood vinegar, comprises: crushing dried soybean straw, cotton straw and stem of castor oil plant; mixing the pulverized soybean straw, the pulverized cotton straw and the pulverized stem of castor oil plant to obtain a mixed straw; spraying a clean water on a surface of the mixed straw; extruding the mixed straw in an extrusion cylinder of a straw pellet machine; maintaining the temperature of the cylinder; pyrolyzing the straw in the cylinder to collect charcoal and gas products, and then condensing the gas product to obtain crude wood vinegar; and vacuum distilling the vinegar.    USE - The method is useful for preparing wood vinegar (claimed) that is useful as a water soluble fertilizer for improving agricultural crop growth.    DETAILED DESCRIPTION - Preparing wood vinegar, comprises: crushing dried soybean straw, cotton straw and stem of castor oil plant to a size of 1.5 cm; mixing the pulverized soybean straw, the pulverized cotton straw and the pulverized stem of castor oil plant at a ratio of 1:1.2-1.7:1.5-2 to obtain a mixed straw; spraying a clean water on a surface of the mixed straw, where the water content of the mixed straw is 16-27%; extruding the mixed straw in an extrusion cylinder of a straw pellet machine; maintaining the temperature of the cylinder at 10-35 degrees C for 15-40 hours; pyrolyzing the straw in the cylinder at a pyrolysis temperature of 560-660 degrees C for 2-5 hours to collect charcoal and gas products, and then condensing the gas product to obtain crude wood vinegar; vacuum distilling the crude wood vinegar at 100 degrees C for 8-10 hours; and distilling the distillate under atmospheric pressure for 3-5 hours to collect the condensate that is purified wood vinegar. The straw pellet machine comprises the cylinder containing a barrel provided with a ring mold plate. A through hole is formed in a ring mold plate. An upper surface of an annular die plate comprises three annular grooves. Each of the annular grooves is located in different horizontal plane. A discharge opening is formed at angle of 360 degrees with respect to each of the annular grooves. The through hole is formed mounted within a support base. The support base is mounted in the middle of a shaft hole of a longitudinal drive shaft. The drive shaft is driven by a motor, and passes through an upper end of the support base. A vertical drive shaft is provided in a support shaft. Ends of the support shaft are connected to a first sleeve and a second sleeve. The first sleeve is mounted on a first rolling disk. The second sleeve is mounted in the second rolling disk. The support shaft further comprises a first rolling plate, a second rolling plate, a third rolling plate, a fourth rolling plate, a fifth rolling plate and a sixth rolling plate. An outer diameter of the first rolling plate is smaller than an outer diameter of the second rolling plate. The outer diameter of the second rolling plate is smaller than an outer diameter of the third rolling plate. An outer diameter of the fourth rolling plate is smaller than the outer diameter of the fifth rolling plate. The outer diameter of the fifth rolling plate is smaller than an outer diameter of the sixth rolling plate. An inner surface of the annular grooves of the upper mold plate has a stepped distribution. An upper end surface of the first annular groove is higher than an upper end surface of the second annular groove. The upper end surface of the second annular groove is higher than an upper end surface of the third annular groove. An inclined connection is formed between the first annular groove and the second annular groove and between the second annular groove and the third annular groove. The first, second and third annular grooves have a depth of 15-25 mm.