The determination of the optimal maintenance time can ensure the reliability of the system and reduce the maintenance cost. In this article, to improve the performance of optimal maintenance timing (OMT) determination for complex electromechanical equipment (CEE), a new OMT model with quality factor is developed based on evidential reasoning (ER), where the influence of historical maintenance and historical storage is addressed by introducing maintenance and storage (MS) impact factors (IFs) into the fusion process of ER (ER-MS). In ER-MS, maintenance and storage IFs are accurately defined and then participate in the calculation of evidence reliability. The newly developed ER-MS method aims to handle three problems that exist in engineering practice for OMT determination method for CEE: difficulty in establishing an accurate mathematical model, the influence of historical maintenance and historical storage, and performance analysis of the established model. This article also puts forward a quality factor definition method that combines evidence weight and evidence reliability, where the quality factor turns out to be reasonable. Then, to quantitatively analyze the influence of historical maintenance and historical storage on the performance of CEE, sensitivity analysis of historical factors is carried out based on the transparency and traceability of the ER-MS method. The sensitivity factors obtained can provide an improvement basis for the use and storage of CEE. A case study for an inertial navigation system (INS) is conducted to illustrate the effectiveness of the new OMT model.