Abstract: Optimization of fault-tolerant workspace for a 7R redundant manipulator in the condition of single-joint locking was studied systematically. Firstly, the concept of conversion ratio was introduced based on the change rule of the fault-tolerant workspace volume and the relationship consideration between fault-tolerant workspace and locking workspace. And then the redundant manipulator's fault-tolerant workspace was optimized by determining suitable joint's locking range. The study results show conversion ratio increases initially and decreases afterwards as the locking angle increases by two kinds of value method. Meanwhile, the locking angle of highest conversion ratio is also determined.