The synthesis process of phenyl dimethylene diisocyanate belongs to the field of isocyanate synthesizing technology. In inert liquid medium, phenyl dimethylamine or its hydrochloride or carbonate reacts with bis(trichloromethyl) carbonate at the temperature of -20 deg.c to -60 deg.c, before heating and reflux reaction for 0.5-5 hr to synthesize phenyl dimethylene diisocyanate. The amount of bis(trichloromethyl) carbonate is 0.5-15 times that of phenyl dimethylamine or its hydrochloride or carbonate. The present invention has stable solid crystal materials, and is environment friendly and suitable for industrial production.