In the process of finishing mill forging，the hydraulic signal may have been affected by the variation in thickness and width of the steel. It results in a hydraulic impact of hydraulic system. The hydraulic signal of finishing action of each sequence period was analyzed. A large number of sample data were collected, the change trend of hydraulic signal was classified and fitted to explore the influence factors of hydraulic signal in the finishing process. Through the generalized correlation analysis, the cause of hydraulic impact fault and its mechanism were concluded. The control system was optimized correspondingly, so that the instability and product quality issues caused by the hydraulic shock of hydraulic system were solved.