The utility model relates to the field of ship lifts, in particular to a speed reducer for a ship lift. According to the speed reducer for the ship lift, a group of four-stage parallel axis speed reducing transmission mechanisms and a group of right-angle alternating axis synchronous transmission mechanisms are installed inside a shell body, and therefore the purpose of power transmission distributary and synchronous working in the ship lift is achieved through the combination of the four-stage parallel axis speed reducing transmission mechanisms and the right-angle alternating axis synchronous transmission mechanisms. The speed reducer for the ship lift is compact in structure, small in overall occupied space, stable in operation, and convenient to use and maintain.