To address the problems of low accuracy and poor reliability in many existing automatic measurement methods of shield tail clearance, a new visual measurement method based on double line laser reference ruler is proposed inthis article. The observation datum is constructed by two parallel narrow linear lasers, and the key features of shield shell and segment are captured by traditional image processing technology. Combined with the ruler back calculation error compensation technology, the high-precision real-time calculation of the measured value of shield tail clearance during shield tunneling is realized. Field experiments show that the repeatability measurement accuracy of the proposed method is better than 1.2 mm, the absolute measurement accuracy is better than 2 mm, and the measurement error is less than 1.5 mm. This method can realize the automatic real-time accurate measurement of shield tail clearance and has high reliability. It has been applied inmany subway tunnel construction sites in China.