Keeping ligament strain at an appropriate range is beneficial for avoiding unexpected injuries and enhancing treatment efficacy. This study proposes a new trajectory determination method specifically for the robot-assisted ankle ligament rehabilitation. The input of this method is a set of strain constraints of certain ligaments and the output is the detailed training trajectory. Simulations were conducted with two cases (one-ligament injury and three-ligaments injury). While this method has not been experimentally tested, on condition of an accurate ligament kinematics assessment, ligament strain can be guaranteed to be within the specified range following the derived trajectory. This method can help design injury-specific treatment protocols and has potential in improving the effectiveness of robot-assisted ankle rehabilitation. Future work will verify the validity and the practicality, and consider the improvement of the method.