The traditional phase difference method finds it difficult to extract detection features and the hardware circuit structure is complex in the DC residual current protection method. Thus a new phase difference method for DC residual current detection based on the characteristic of the duration of the exciting current negative half wave in a period is proposed. In this method, the difference between the duration of negative half wave of excitation current before and after the appearance of residual current is detected to detect DC residual current. First, the excitation current is rectified by half wave, and the duration of negative half wave is converted into the duration of zero amplitude excitation current. Then, the difference between the duration before and after the appearance of residual current is detected to achieve the purpose of measurement. Based on this method, the hardware circuit of a DC residual current transformer is designed, and the effectiveness of this method is verified by experiment. This work is supported by the National Key Research and Development Program of China (No. 2018YFB0904700).