Firstly, the methodology for building up the model for the static strength and fatigue strength reliability of cylindrical helical spring is analyzed, and the computational formulas and distributions of the stress and strength are derived. Secondly, with the individual characteristics of mechanical optimization design and reliability, taking the spring's mass as the objective function and based on the traditional constraints and reliability constraints of the spring, the mathematic model of reliability optimum design of cylindrical helical spring is established to make full use of the materials and consider the reliability of the part. At last, combing the optimal tools with the optimization model, the proposed method is applied to a practical example, the results of the reliability optimization and the traditional optimization are compared and analyzed, which show that the proposed method of the reliability optimization designing is practical, reliable and can satisfy the demands of designing to the full extent.