In order to improve the positioning accuracy and reliability of the navigation system in complex underwater environment, a strapdown inertial navigation system (SINS)/doppler velocity log (DVL)/ultra-short baseline (USBL) tightly coupled algorithm is proposed based on a centralized Kalman filter. A new 26-D tightly coupled measurement model is constructed. Different from the traditional loosely coupled algorithm, the proposed model directly integrates the original measurement information of each sensor, instead of the navigation information such as position and velocity. Simulation and experiment results show that the proposed algorithm can make the best use of the only original information when the sensor cannot output the correct position and velocity in case of poor measurement conditions. Compared with the loosely coupled algorithm, the proposed algorithm has better positioning accuracy and higher reliability.