Reliability evaluation plays a more and more important role in nuclear engineering, aviation industry, and so on. Due to the special integrity of a control system, traditional reliability methods, such as the reliability block diagram and fault tree analysis, are not proper to make a reasonable and accurate reliability evaluation on a control system. However, a systematic methodology to predict performance reliability of a control system is rarely reported. In this article, a methodology of performance reliability based on the performance requirement for an automatic control system, not just limited to the stability reliability or probabilistic robustness evaluation, is first proposed. A solution to evaluate control performance reliability is introduced with analytical approximation approach called first-order second-moment method. Also, a robust iterative algorithm is adopted to overcome the bifurcation and chaos problems encountered when searching the most probable point for highly nonlinear limit state function. An elaborate example based on a servo motor control system is presented to calculate performance reliability of the settling time of the control system with first-order second-moment method. Its numerical approximation is demonstrated to be accurate through Monte Carlo simulation.