The invention discloses a method used for preparing beta-methyl epichlorohydrin in Sc-CO2 system. According to the method, reaction raw material hydrogen peroxide is mixed with supercritical carbon dioxide solvent (SC-CO2) treated via refrigeration, pressurization, and preheating in a mixer until an appropriate input concentration is achieved; pH value is adjusted with a buffering agent ammonium carbonate solution; a certain amount of beta-methylallyl chloride, which is taken as both a raw material and a cosolvent, is added; an obtained mixture is delivered into a tubular fixed bed layer reactor containing a Ti-MWW titanium silicalite molecular sieve catalyst for epoxidation; and an obtained product is subjected to rectification so as to obtain beta-methyl epichlorohydrin. According to the synthesis process, operation is convenient, cost is low, yield is high, energy is saved, no environment pollution is caused, effective utilization of greenhouse effect gas carbon dioxide is realized, the method possesses significant importance on environment protection, and is a typical atom economy synthesis technology route.