The invention provides a voltage sag transition process simulation device and a method. The device comprises an input switch, an input transformer, a connection inductor, a power unit, an output inductor and an output switch. The method comprises steps of determining an equivalent network module of a to-be-simulated system and a short-circuit default type of the to-be-simulated system; determining default points, to-be-observed voltage sags and default duration time of the equivalent network module; applying the equivalent network the symmetrical component method to determine voltage steady-state components before and after defaults; calculating the transient-state components in the voltage sag transition process according to the state equation of the equivalent network module; and overlaying the steady-state components and the transient-state components to form pulse width modulation signals and adding the pulse width modulation signals on the inverted side of the power unit to allow the device to output the required three-phase voltage sag waveform. According to the invention, any voltage sag depth, voltage sag duration time and complex phase position and amplitude relations between three-phase voltage in the voltage sag process of a power grid can be simulated and reference can be provided for precise simulation of voltage sags in the power grid.