Through the sensitivity analysis of large-scale grid reliability, we can determine the key equipment influencing the power supply reliability of the system, and thus effectively find out the bottlenecks limiting the system reliability. In this Paper, the circuit outage model is established, and the former complex large-scale grid with various equipments is divided to two levels: circuit and grid which can be calculated and analyzed in order. Therefore, the complexity of sensitivity computing of large-scale grid is reduced effectively. The circuit equivalent model and grid reliability indicator are deduced by the min. cut set algorithm, based on which various sensitivity indicators of grid reliability indicators against original reliability parameters of equipment are acquired. The routine computing and analysis of National Electric Power Facilities have fully verified its correctness and practicality. The correctness of the algorithm and procedure is described in this Paper through the actual system of Northwest Grid.