Nowadays, the navigation systems, such as phone-based and vehicle navigation, are developing, and techniques of GNSS/INS loosely and tightly coupled navigation are under widely used. Compared with loosely coupled navigation, the tightly coupled navigation shows more and more superiorities in urban canyons, because it can provide continuous integrated navigation solution even with less than four satellites. Conventional tightly coupled navigation only use pseudo range and Doppler observations, while the time-differenced carrier phase (TDCP)/ Inertial navigation system(INS) tightly coupled navigation uses an extra TDCP observation;this can provides higher attitude accuracy than the conventional tightly coupled navigation. However, the mathematical model of the conventional TDCP/INS tightly coupled navigation makes difference between the reference epoch and current epoch, this creates poor real-time. To improve the accuracy and reduce the computation burden, this paper presents a modified TDCP/INS tightly coupled navigation method, infers a modified mathematical model of TDCP, which can make difference between the current 0.1s epoch. The experimental results show that, compared with the loosely coupled navigation, the modified TDCP/INS tightly coupled navigation method provides higher accuracy of position, velocity and attitude. As expected,compared with the conventional tightly coupled navigation, the attitude accuracy is significantly improved, meanwhile the accuracy of position and velocity are slightly improved. In the face of few observable satellites and turning, the modified TDCP/INS tightly coupled navigation method shows a clear advantage.