In order to dynamically detect the hydrological data of oceans and lakes, an unmanned semi-submersible vehicle was designed. Based on the design idea of modularization, the unmanned semi-submersible vehicle is divided into four functional modules:submergence main hull, surface floating body, control module and lifting mechanism. The main hull structure was designed into a catamaran, composed of left, right submarines, according to the functional requirements, each submarine is divided into five functional modules:dome, sensor module 1,battery module, sensor module 2 and electric motor module; The surface floating body is the shape of the ship, the control module is embedded in it; The lifting mechanism adopts stepper motor and screw rod to adjust the depth of the main hull. Based on the main size of the unmanned semi-submersible vehicle, the unmanned semi-submersible vehicle is checked for stability. Using FLUENT software to analyze the model, calculate the required power, and select the appropriate motor. The experiment shows that the design of unmanned semi-submersible vehicle is stable in the water and has the function of advancing and changing the depth of the main hull.