To accurately calculate the condition-based maintenance cycle for a protection relay system, and to make it convenient for the operational staff to prepare the maintenance schedules, a new condition-based maintenance strategy is developed based on failure probability analysis. Given the historical failure data, a fitted model is established. By considering the influence of short- and long-term factors, a real amending failure probability model for a protection relay system is proposed. Then, in accordance with engineering experience and some related guides, the maintenance cycle can be calculated by setting the accumulated failure probability threshold. The simulation results testify that the proposed maintenance strategy can provide a more practical maintenance cycle for operational staff and increase the engineering applicability. This work is supported by the Science and Technology Project of State Grid Corporation of China (No. 521200190081).