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Knowledge of the disturbance of the concerned bus is essential to make decisions to mitigate voltage sags. The most commonly used method is to estimate the state of voltage sag in the absence of a monitor. However, the traditional state estimation method, which is designed for rectangular voltage sags, is difficult to apply to multi-stage voltage sags (MSVSs). This study proposes an MSVS state-estimation method based on event-deduction models. First, we propose a method to identify the causes of MSVSs. Second, we present a method to calculate the matrix of relay protection trips (MRPT), which corresponds to the fault clearing time of the relay protection and a method to calculate the fault power line set to reduce the computing workload of the MRPT-based estimation method. Third, we propose a method to calculate the impedance matrix of a power system under different disturbance events, resulting in different stages of the MSVS and an event-deduction model based on this impedance matrix to deduce these events. Furthermore, we present a model to estimate the state of voltage sag for MSVS based on the event deduction result. Finally, we use the IEEE-118 system to validate the proposed method.