Abstract Space manipulators are usually used to assist or replace the astronaut to execute on-orbit tasks. However, due to the burdensome tasks and harsh space environment, free-swinging joint failure is prone to happen, causing the task to be unable to continue. Once the joint failure occurs, the base and free-swinging joint belong to underactuated units, so their trajectories have to be controlled by the active joint. To execute on-orbit tasks successfully after the joint failure occurs, a fault tolerant motion planning and control method (FTMPC) of the space manipulator based on controllable degree is proposed. First, based on the dynamics model of space manipulators, the dynamics coupling relationship between the active joint and the underactuated unit is established. Second, according to the inertia matrix, Coriolis and centrifugal terms of the dynamics model, a controllable degree index of the active joint over the underactuated unit is constructed. Third, according to the specific task requirement and controlled object, the FTMPC based on the controllable degree is designed. The FTMPC is applied according to the number of active joints and underactuated units and whether there is a dynamic singularity during the task execution. The simulation is carried out with a 7-DOF space manipulator, verifying the correctness and effectiveness of the FTMPC. The method has two advantages: 1) Coriolis and centrifugal terms are both considered for the first time in evaluating the control ability of active joints, making the evaluation result more in line with the motion characteristics of space manipulators; 2) A FTMPC based on the controllable degree is proposed, completing the task without dynamic singularity or huge torque on the active joint.