Reliability evaluation of microgrid must consider the time-varying operation modes due to the fluctuation of load and renewable energy sources. This paper presents a reliability evaluation model of microgrid by deducing an analytical formulation of reliability parameters in different operational modes, which can be used to accurately consider the transfer characteristics between different operational modes. The RBTS-BUS6 test system is used as an example to demonstrate the feasibility and practicability of the proposed model. The results of the test system indicate that the strategy “diesel generating unit continuous output” improves the reliability of microgrid and has a bigger effect on the load points of external network, whereas the strategy “diesel generating unit in standby” has a better environment and economic benefits. A larger maximum discharging power of a battery is a useful and effective measure to improve the reliability of a microgrid.