The rated life of an electromagnetic relay is the key to meet the requirements of the whole machine, which has a direct influence on the reliability of the system. According to the large endurance discrepancy between different batches, we analyzed the mechanism that affects the rated life of electromagnetic relays and found that the life failure of electromagnetic relays is mainly caused by contact area, contact pressure, contact gap, and the material of contactors. Without changing the structural design and the materials of contactors, the test design instructed by design of experiment (DOE) for the related influencing factors was carried out to improve the rated life and its consistency, and obtained reasonable contact gap, constant contact pressure, and overrun. Besides, the contact gap, constant contact pressure, and the control range of overrun were optimized by using the tolerance design to enhance the consistency of rated life. The result indicates that the DOE test can disclose the optimization scheme of the factors that influence electromagnetic relays and the control range of the influencing factors can be optimized through tolerance design, which can effectively enhance the rated life and its consistency, thus improve product quality.