Abstract: Aiming at the trajectory tracking task, the mathematic relationship between motion reliability and positional accuracy of the robot is described. Combined with the analysis of factors having effect on the positional accuracy, a common model for motion reliability of the robot in the trajectory tracking task is established. Then with the introduction of the approach of equivalent extreme value, the solution of the motion reliability model is simplified from multidimensional integral to onedimensional integral. Based on an eight degrees of freedom modular robot, simulations are carried out for a particular trajectory tracking task, of which the motion reliability is evaluated quantitatively. Considering the same errors the factors having effect on the motion reliability of the robot are analyzed indepth. As a result, the conclusion can be taken as the theoretical foundation for the optimal control of trajectory tracking tasks.