To improve the performance of a cable-driven parallel mechanism（CDPM） for on-orbit assembly through by changing its stiffness， a variable stiffness control algorithm was proposed. First， the complete stiffness model of the CDPM was established. Secondly， the influence of tension distribution on the stiffness in different directions was analyzed and studied. Then， considering the safety margin of the vector λ ， which affected the tension distribution， a variable stiffness control algorithm for the CDPM was established. The algorithm eliminates the coupling stiffness that has a small effect on the overall stiffness in the stiffness model， improves the calculation efficiency， and can assign weights to control the stiffness in different directions with different prioritypriorities. Finally， the effect of the variable stiffness control algorithm was verified through a simulation and an experiment. Taking the screwing operation as an example， the simulation results show that when a disturbing force is applied to the moving platform， the tension distribution obtained by the variable stiffness control algorithm can reduce the vibration in the weak direction by 35%. The prototype experiment results show that after the stiffness control of the CDPM prototype， the measured value of the stiffness has a strong fit with the theoretical value， and the errors for these values are 16.3% and 14.6%， respectively. The proposed variable stiffness control algorithm can improve the performance of the CDPM by changing its stiffness.The algorithm has strong practicability and can be used for different types of redundant CDPMs.