The invention discloses a multi-rate simulation method and a multi-rate simulation device for a power-electronizing power system. The method comprises: dividing the power-electronizing power system into a slow subsystem and a plurality of fast subsystems; establishing equivalent models respectively; setting simulation parameters; performing power flow calculation on the power system to obtain a steady-state initial value of the power system; establishing interfaces between the plurality of fast subsystems and the slow subsystem, and establishing a time-varying Thevenin equivalent circuit and a time-varying Norton equivalent circuit; and obtaining interface parameters according to the established time-varying Thevenin equivalent circuit and the established time-varying Norton equivalent circuit, and parallelly solving node admittance equations corresponding to the equivalent models of each fast subsystem and the slow subsystem until simulation ends up so as to obtain an electromagnetic transient state simulation result. By the simulation method provided by the embodiment of the invention, the non-linear dynamic characteristics of the internal part of an alternating current system or a direct current power grid can be maintained; meanwhile, the simulation efficiency is obviously improved relative to a single step length result.