The invention discloses a rope drive decoupling mechanism based on a gear train and a decoupling method thereof, and particularly relates to a multi-joint rope drive series mechanical arm system. The mechanism comprises a fixed wheel, a follower wheel, a driving wheel, a guide pulley and the planetary gear train. The driving wheel is connected with a joint connecting rod. The fixed wheel is connected with a joint base. The follower wheel is driven by the planetary gear train, and the rotating speed of the follower wheel is 1/2 that of the joint connecting rod. A drive rope of a rear end joint is wound according to the forward direction and reverse direction of a wire groove in the follower wheel, so that drive rope displacement changes caused by rotation of the joint connecting rod and displacement changes caused by rotation of the follower wheel offset. The drive rope passing through the decoupling mechanism and the joint connecting rod have no relative displacement, and motion decoupling of the rope is achieved. The decoupling mechanism can work within the range of 0 DEG C to 300 DEG C, and is wide in work range and simple, compact and reliable in structure. The rope motion decoupling mechanism and a rope drive joint are mounted in a matched manner, modularization can be achieved, and the application range is wide.