The invention discloses a three-branched-chain four-freedom-degree robot mechanism achieving three-dimensional translation and one-dimensional rotation. The mechanism comprises a fixed frame, a moving platform, a first branched chain, a second branched chain and a third branched chain. The first branched chain, the second branched chain and the third branched chain are arranged between the fixed frame and the moving platform. The three branched chains each comprise a close frame bar and two parallel and equilong far frame bars. The third branched chain further comprises a rotating fork, and the rotating fork is rotationally connected with the fixed frame. One end of each close frame bar is connected with the fixed frame or the rotating fork, the other end of each close frame bar is spherically hinged to the corresponding far frame bars, and each far frame bar is spherically hinged to the moving platform. The moving platform comprises a main platform body and an auxiliary platform body which are rotationally connected. According to the mechanism, three-dimensional translation and one-dimensional rotation four-freedom-degree motion is achieved through the three branched chains, and the manufacturing cost can be reduced easily.