Abstract: An underwater sonar device and an underwater detecting system. The underwater sonar device comprises a main body, a propeller, a detector and a hydrofoil assembly. The main body is an axisymmetric structure. The propeller, the detector, and the hydrofoil assembly are disposed on the main body. The detector is configured to detect and image an underwater target. The propeller is configured to drive the main body to move along a longitudinal direction and a vertical direction, and control a pitch angle, a roll angle, and a yaw angle of the main body. The hydrofoil assembly is disposed at a back of the main body, and is configured to adjust an included angle between the hydrofoil assembly and the longitudinal direction of the main body automatically based on water resistance on the hydrofoil assembly to keep the sonar device navigating at a fixed depth.