The invention aims to provide an underwater robot locking mechanism. The underwater robot locking mechanism comprises a fixed seat, a hydraulic cylinder, a guide rail, a spring shaft and a clamping mechanism, wherein the guide rail is fixed above the fixed seat; the hydraulic cylinder is mounted below the fixed seat; a slide block which is matched with the guide rail is arranged at the inner side of the guide rail; the clamping mechanism comprises a clamping plate and a guide plate; the guide plate is fixed below the clamping plate; the slide block is fixed with the guide plate; the end part of a piston rod of the hydraulic cylinder is connected with the spring shaft; a spring is arranged on the spring shaft in a sleeving mode; a hole is formed in the clamping plate; the upper end of the spring shaft penetrates through the hole of the clamping plate and is connected with a positioning sheet; a bulge is arranged at the lower end of the spring shaft; one end of the spring is resisted against the clamping plate; and the other end of the spring is resisted against the bulge of the spring shaft. According to the underwater robot unlocking device, an underwater robot is locked by virtue of force produced by spring compression, so that rigid contact is turned into elastic contact, damage risks to the underwater robot are reduced, long-time locking to the underwater robot can be realized by the spring compression, and other complex hydraulic locking devices are not required.