The invention discloses a shading device for a vehicle-mounted road crack detection system based on a symmetrical four-bar mechanism, and aims to solve the problem of automatic shading of a vehicle-mounted road detection system. The device is composed of a base (1), a cover plate (2), a cover plate rack (3), a connecting rod seat (4), a female connecting rod (5), a male connecting rod (6), a cover plate female connecting rod (7), a rack seat (8), a rack (9), a cover plate male connecting rod (10), a spur gear (11), a motor (12), a camera (13), a cover plate shaft (14), a cover plate shaft sleeve (15) and a camera seat (16). The provided shading device for the vehicle-mounted road crack detection system based on the symmetrical four-bar mechanism is simple in structure, simple and convenient in operation, easy to install, low in cost and reliable in performance.