To solve the problem of driving different length of public switched telephone network (PSTN) subscriber line, a method of automatic measurement of subscriber line length and switching circuit is presented. By the construction of subscriber line model, the line model is simplified according to the type of signal. A DC algorithm is used to measure the length of the subscriber line. Circuit and parameter profiles are switched automatically according to different line state of“ring" and "feed". Experiments show that the proposed method can reduce the power consumption of the subscriber line interface circuit (SLIC), extend the subscriber line distance, and enhance the subscriber’s experience.