According to geological section data for the Tongbai fault and the seismic survey line LFZS1 arranged by the Geophysical Exploration Center of the China Earthquake Administration, we determined the Tongbai fault to be the northern boundary fault of the Langgu Sag, which is composed of two parallel faults-a main fault and a branch fault. We then conducted a shallow seismic exploration of the Tongbai fault. The shallow seismic exploration profiles reveal that the main fault strikes nearly EW and the buried depth of the upper break point is about 200 m. The latest activity of the main fault occurred in the early Pleistocene. The buried depth of the upper break point of the branch fault is about 150 m and the latest activity occurred in the middle Pleistocene. This conclusion provides reference information for seismic safety considerations of important buildings and urban construction planning around the Tongbai fault.