In the field of distribution automation system, feeder terminal unit (FTU) is always applied to acquire signals of distribution networks, and to monitor distribution networks and control equipments. However, due to the existence of sensors, sampling circuit or outside interruptions, errors are inevitably introduced into sampling values when acquiring voltage and current, causing acquired signals inaccurate and influencing accuracy of measurements of distribution network parameters. Residual DC is one of introduced errors which can"t be ignored. In this paper, a residual DC filter algorithm during AC sampling based on FTU is proposed. This algorithm is able to filter out residual DC from acquired instantaneous sampling values, making data more precise and ensuring measurements of distribution network parameters more accuracy. MATLAB was applied to simulate this algorithm and results showed a good effect of filtering out residual DC from AC sampling values, which is of strong practical value.