The invention discloses a mechanical device for tunnel prefabricated piece assembly. The mechanical device includes a moving bottom disc, a bottom bracket, an upper bracket and a top bracket; the bottom portion of the bottom bracket is installed on the moving bottom disc in a lifting mode through a lifting mechanism, and the top portion of the bottom bracket is provided with a first direction guide rail and a first direction telescopic device; the bottom portion of the upper bracket is installed on the first guide rail and can move along the first direction guide rail under the action of the first direction telescopic device, and the top portion of the upper bracket is provided with a multi-direction rotating oil cylinder group; and the top bracket is installed on the multi-direction rotating oil cylinder group and can perform space rotation in at least three different directions under the action of the multi-direction rotating oil cylinder group. According to the mechanical device fortunnel prefabricated piece assembly, the limit space in a tunnel is overcome, and original tunnel structural facilities are adopted; and the mechanical device for the tunnel prefabricated piece assembly adopts various high-precision translational movement structures and a plurality of hydraulic oil cylinders to control, and the high-precision assembly of prefabricated pieces in the tunnel can beachieved.