A synthesis method of an intermediate chloroacetonitrile of a phentolamine mesilate drug comprises the following steps: adding 6.3 mol of 1-chloro-2, 2-dihydroxyethylamine, 5.6 mol of cuprous chloride, 7.3-7.6 mol of 2-aminobromobenzene solution and 300 ml of cyclohexane into a reaction container equipped with a stirrer, a thermometer and a dropping funnel, controlling the stirring speed at 150-190 rpm, raising the temperature of the solution to 90-98 DEG C, reacting for 5-7 h, reducing the temperature of the solution to 70-76 DEG C, continuing reacting for 80-120min, reducing the temperature of the solution to 10-16 DEG C, standing for 8-10 h, separating out a solid, filtering, washing by using salt solution, washing by using acetonitrile, washing by using ethyl acetate, dehydrating by using a dehydrating agent, performing reduced pressure distillation, collecting a cut fraction with the temperature of 100-108 DEG C, and recrystallizing in dimethylamine to obtain the crystal chloroacetonitrile.