The reliability tracking is used to determine the distribution relationship between elements and nodes or system index by the use of tracking thought. The weak part could be identified by comparing the contribution of component reliability to system reliability. The technique of determining the contribution of each component to the system unreliability can be used to identify the system weak parts. Based on unreliability-tracing principle, this paper analyzes the contribution of different components to the whole system unreliability and identifies the weak parts which impact system reliability significantly under four typical operation modes; and then proposes the measures to improve the reliability of power supply; the system reliability is evaluated after implementing improvement measures, which prove the validity of the proposed model.