In order to investigate the influence of laser process on the cleaning quality of oil contamination layer on the surface of steel parts, a laser decontamination cleaning test was carried out on a Q235 steel plate specimen by using a pulsed laser, and the cleaning quality and damage of the specimen surface were detected. The mechanism and effect of the laser beam on the oil contamination layer and substrate steel in the cleaning process were theoretically analyzed, a laser thermal effect model was established, and the influence of the degree of defocusing on the surface temperature of the substrate through simulation calculation and test were analyzed. The removal effect of oil contamination on the surface of Q235 steel parts and the damage effect on the surface of steel parts under different defocusing degree in the laser cleaning process were respectively verified by the test. The results show that the optimal process parameters of laser cleaning of oil contamination layer on the surface of Q235 steel parts is that the defocusing degree is 5 mm, and the average power is 4 W. The influence of defocusing degree on the cleaning effect is much bigger than that of the laser power factor. This study provides some reference for the oil treatment process and laser cleaning of metal surfaces.