The invention provides a preparation method of a photoinitiator TPO, comprises the following formula: 22.5% of diethyl aniline, 7.5% of absolute methanol, 34% of chlorodiphenylphosphine, and 36% of 2,4,6-trimethyl benzoyl chloride. The method comprises the following steps: 1, stirring and uniformly mixing diethyl aniline and absolute methanol; 2, warming to 20 DEG.C, slowly dropping chlorodiphenylphosphine, preserving heat for 2 hours; 3, pumping out the material, and filtering for later use; 4, uniformly mixing the 2,4,6-trimethyl benzoyl chloride; 5, warming to 80 DEG.C, slowly dropping the material for later use in the step 3, and then preserving for 1 hour; 6, carrying out suction filtering by using a suction filtering groove to obtain the photoinitiator TPO. The process is simple, the flammable and combustible solvent such as methylbenzene is unnecessary to use in the reaction process, the preparation method is safe, reliable and in favor of energy-saving and environment-protecting.