This paper proposes a bathymetric simultaneous localization and mapping (SLAM) approach for underwater vehicles to conduct mission in the region with no high-resolution prior seabed terrain map while requires long-endurance autonomous navigation especially in polar region. Because of the rapid attenuation of electromagnetic waves in water mediums and insurmountable defects of regular sonar equipment, satellite navigation and acoustic methodologies are impractical in this scenario. To enhance the performance of navigation this paper chooses multibeam sonar as external sensor to detect the seabed terrain while employs the internal inertial navigation system to provide state information of vehicle to form one integrated navigation solution which not only perceives the position of vehicle but also charts the seabed terrain map of traveled area. The framework consists of multibeam sonar bathymetric method, terrain interpolation, submap processing strategy, path correction technique, and bathymetric SLAM mechanism. By using one real regional seabed terrain map of Arctic Ocean, one deepwater MBS model, and one trajectory of INS, a simulation experiment is conducted and verifies the performance of the proposed approach in detail.