Abstract: We calculate the focal depth of the Badong M4.3 earthquake sequence and analyze the depth error using the data from the Three Gorges telemetry network and the mobile station. The results show that the sequence of the focal depth is mainly distributed in the 3-5 km; mobile station observation can obviously improve the accuracy of the sequence determination of focal depth, and focal depth error is reduced from 1.8 km to 1.1 km. It is considered that the focal depth in the Three Gorges telemetry network (especially after setting up the mobile station) can be determined by direct wave method.