As a new structure of the distribution network， the microgrid can switch from grid-connected mode to grid-off mode flexibly. This feature can make consumers in microgrid get better power supply and guarantee the power supply for important loads against power off in extreme conditions such as earthquakes， snowstorms and the collapse of the large grid. Fault tree analysis is used to develop the reliability model for the microgrid in grid-off mode and calculate the failure rate in fixed time with Monte Carlo simulation method. In this way， the uninterruptible power system（UPS） capacity of microgrid in extreme conditions is assessed. Take an actual microgrid system for an example， the impact of the various components of microgrid on the UPS capacity is analyzed， which is helpful to making some recommendations on the microgrid construction.