The mine main ventilation fan, as the core of mine safety operations, consists of two cold storage units of the same type of equipment, which often need to be switched and maintained periodically to ensure its efficient operation. For such systems, the characteristics of alternating equipment operation lead to system failures that are affected by the coupling of equipment states, and different repairs correspond to different effects, times and costs and have different impacts on the subsequent operation of the system. Therefore, the failure analysis of the system and the reasonable maintenance strategy are crucial for the system's reliable operation. This research takes the Mine main ventilation fan as the research object. Firstly, based on the analysis of structure and failure mode, the offline switching maintenance strategy is defined, and the concept of maintenance degree is introduced to model the effect, time, cost and frequency of maintenance in a unified way. Secondly, the stochastic failure model of the system under the influence of the offline switching maintenance and the maintenance degree is deduced. Then, a cost rate model is constructed in the finite time domain to optimize the maintenance period and the maintenance degree. Finally, the validity of the model is verified by numerical experiments. The results show that the offline switching maintenance strategy considering the maintenance degree can effectively reduce the operation and maintenance cost of the mine main ventilation system. Meanwhile, the change of the scrap cost of components has a more sensitive impact on the optimal cost rate of the system, and the manager can further improve the system's economic efficiency through the refinement of the management of the scrap cost of the equipment.