The invention discloses a circuit breaker state online diagnosis method and a system based on PMU (Phasor Measurement Unit) data, which relate to the technical field of power system circuit breaker protection. A grey target theory is introduced to circuit breaker state diagnosis, the irrationality of adopting a weight average algorithm to calculate the approaching degree by the traditional grey target theory is improved, a hierarchy-entropy weight combined weight calculation method is put forward, the influence weight of each index for an evaluation result is solved, advantages of the two aretaken to remedy disadvantages of each, both subjective and objective factors are considered, defects of a single assignment method are avoided, weight determination is thus more rational, the safe andreliable operation is very important for improving reliable power supply of the power grid, and a basis is laid for realizing state maintenance for the circuit breaker.