In the AD sampling process of Siemens S7-1200PLC, sampling data jitter is caused by external interference and sensor sampling data disturbance. Based on the principle of Least Mean Square (LMS), an improved weighted average digital filtering algorithm is designed in this paper. In 1200PLC, the corresponding filter algorithm program is compiled by SCL language to filter the sampled data of AD. An experiment is done on the AD sampling under a strong interference environment, and the sampling data is stable and accurate, proving that the digital filtering algorithm is efficient and reliable.