To improve the calculation efficiency of reliability assessment for power system, a load shedding model of reverse equivalent pairing based on power flow sensitivity is proposed. Firstly, based on the sensitivity in the system's base state, an analytical formula of the sensitivity in fault state is deduced, and thus a fast solution to the trend sensitivity matrix is achieved. Secondly, according to the power flow sensitivity of nodes injection power with respect to overload branches, the selection rule of control nodes for both generation and load nodes is presented. Furthermore, a model of reverse equivalent pairing adjustment is constructed based on this selection rule. A modified formula of power adjustment amount of pairing nodes group is also presented and it eliminates the see-saw phenomenon of overload for normal transmission lines in adjusting process and avoids the power flow verification in traditional methods, which makes the system has a higher state analysis speed. Finally, the effectiveness and fastness are verified by evaluating RBTS, IEEE-RTS79 and IEEE-RTS96 test systems.