The invention discloses a Monte Carlo-based equivalence reliability assessing method. The method includes the steps of inputting whole network basic data, establishing an equivalence network according to a static equivalence method taking sensitivity consistency and element type comprehensiveness into consideration, and calculating all equivalence network parameters. The element operation and load fluctuation magnitude are simulated by using a non-sequential Monte Carlo method to obtain random state of an internal network system. Whether network topology is in a stepout state is analyzed, and whether branch trend or node voltage exceeds limits can be determined through trend calculation. The equivalence reliability of the system is comprehensively evaluated, and compared with the result obtained based on routine hang-up equivalence theory. When used for external network equivalence, the equivalence method ensures comprehensiveness of element types, and takes trend and sensitivity consistency into consideration. The calculation speed of reliability assessment is effectively improved through the method in combination with the non-sequential Monte Carlo method while ensuring equivalence precision.