DC transmission line is the core part of DC power system, and its operation status determines the transmission efficiency of DC system, so it is necessary to accurately measure the voltage and current of DC line. However, due to the problems of insufficient measurement accuracy, long-term drift and high failure rate of DC transformers, there are often abnormal data measured by DC transformers in actual projects, leading to misoperation of control and protection equipment. Since DC systems must deal with faults in a short time, DC control and protection equipment is required to have the ability to identify bad data quickly. In this paper, we propose a method for identifying bad data on DC transmission lines based on moving average method with simple principle and small calculation. After verification by simulation examples, the method has good accuracy and feasibility, and can meet the needs of DC control and protection equipment for real-time and accuracy of bad data identification.