Aiming at the problem of numerical filtering in PLC control system, the basic working principle and mathematical model of RC filter circuit were studied. The method of continuous differential method was adopted to solve the differential equation mathematical model of RC filter circuit. The recurrence relation model of digital filter was deduced, and the numerical filtering program of PLC was programmed. Effectively overcoming the problem of signal instability caused by high frequency interference in the complex electromagnetic environment of the industrial field, good results had been achieved in the application of the actual control system.