The invention relates to a synthesis process for 3,4-dichlorophenyl isocyanate as a diuron intermediate. The steps are as follows: (1) 3,4-dichloroaniline and methylbenzene are added into a dehydration kettle, so that 3,4-dichloroaniline methylbenzene solution is obtained; (2) the internal temperature of the dehydration kettle in step (1) is increased; when moisture in the dehydration kettle is lower than 0.5 percent, the 3,4-dichloroaniline methylbenzene solution in step (1) is poured into a metering tank, phosgene methylbenzene solution is dripped, and phosgene is injected at the same time; (3) after the dripping of the phosgene methylbenzene solution in step 2 is complete, phosgene continues to be injected, and thereby raw 3,4-dichlorophenyl isocyanate is obtained; (4) the raw 3,4-dichlorophenyl isocyanate in step (3) is poured into a thermo-optical kettle, meanwhile, after the temperature of the thermo-optical kettle is increased to 110 DEG C, filtration is carried out, and thereby the 3,4-dichlorophenyl isocyanate is obtained. The synthesis process is easy to operate, and the purity of the prepared 3,4-dichlorophenyl isocyanate is high.