As the construction environment of shield tunneling is always bad, manual measurement is dangerous and indirect measurement methods, i. e. visual sense and laser are of low accuracy. A non-contact embedded direct measurement system with ultra-sonic sensor is explored. Distance measurement unit of embedded ultra-sonic sensor, acquisition unit of signal processor, display unit of shield tail gap measurement are studied and the workflow of the whole measurement system is summarized. With experimental verification and industrial test, it is verified that the measurement system is highly accurate and can provide references for K block selection and attitude adjust, which is helpful for automatic selection of pipe segments and intelligent correction of deviation.