NOVELTY - Acidizing guanidine nitrate aqueous solution to prepare nitroguanidine in continuous flow micro-channel reactor, comprises (i) taking guanidine nitrate aqueous solution and concentrated sulfuric acid, introducing into microchannel reactor, and controlling flow speed of guanidine nitrate aqueous solution and concentrated sulfuric acid; (ii) continuously reinforcing the mass transfer through series of e.g. micro-channel module, collecting product, and cooling; and (iii) flowing the reaction liquid into collector filled with ice, stirring, filtering, washing, re-crystallizing, and drying.    USE - The method is useful for acidizing guanidine nitrate aqueous solution to prepare nitroguanidine in continuous flow micro-channel reactor (claimed).    ADVANTAGE - The method is simple and safe to operate; has high-yield continuous production for nitroguanidine; and produces low environmental pollution.    DETAILED DESCRIPTION - Acidizing guanidine nitrate aqueous solution to prepare nitroguanidine in continuous flow micro-channel reactor, comprises (i) taking guanidine nitrate aqueous solution and concentrated sulfuric acid as reaction material, firstly introducing into a microchannel reactor, controlling pre-setting temperature by an external heat exchanger, using a heat exchange medium comprising heat conducting oil and water, changing the flow control rate of guanidine nitrate aqueous solution and concentrated sulfuric acid according to mol ratio of 1:0.9-1:100, controlling the flow speed of guanidine nitrate aqueous solution to 0.2-10 ml/minute, controlling the flow rate of concentrated sulfuric acid to 0.2-10 ml/minute, where the concentration of guanidine nitrate aqueous solution is 10-60 wt.%, synchronously conveying into reinforced mass transfer module to a mixed reaction via respective metering pump, and controlling the mixing temperature using the external heat exchanger; (ii) after mixing and reacting in the module, continuously reinforcing the mass transfer through a series of micro-channel module and a direct current micro-channel modules, after finishing the reaction, collecting the product from the outlet of the reactor, and then cooling, where the reaction temperature in the microchannel reactor is 40-120 degrees C, and the reaction time is 50-150 seconds; and (iii) directly flowing the reaction liquid obtained from the reactor outlet into a collector filled with ice, stirring while it is liquid, continuously precipitating solid, after the reaction liquid stops flowing, continuously stirring for 5 minutes, filtering, washing the filter cake with water for 2 times, and then re-crystallizing by water at 35 degrees C and drying under reduced pressure for 6 hours to obtain white powder solid, where the yield is 80-85%, and the melting point is 230-232 degrees C.