Huge amount current transformers and power meters are installed in the power system of the Spaceflight Launch Site, and how to find the abnormal ones with large measurement deviation in time is a key technology to ensure the reliability of the ground system. Using power balance theory between input circuits and output, we study the relationships of input-output difference, construct the error prediction model, which is predicted through the least square model. The experimental result based on the real measurement data shows the effectiveness of our model.