The invention relates to a tailskin clearance automatic measuring system for a tunnel boring machine (TBM). The system comprises range finding sensors installed on the inner wall of a tail shield, a PLC communicated and connected with the range finding sensors, and a host computer communicated and connected with the PLC. The range finding sensors directly measure the radial distance, between the outer wall of a segment and the inner wall of the tail shield of the TBM, as a tailskin clearance value, and then transmit the measured tailskin clearance value to the host computer. Workers are not required to directly participate in measurement and the range finding sensors are fixedly mounted on the inner wall of the tail shield, are not liable to move, and ensure that the measured data is the tailskin clearance value in the radial direction and is accurate and reliable. The range finding sensors are divided into a front group and a rear group each of which comprises a plurality of range finding sensors. The range finding sensors in each group are arranged on the inner wall of the tail shield along the circumferential direction. By comparing the tailskin clearance values measured by the two groups of range finding sensors, the installation of the segment in the tail shield can be analyzed accurately.