The invention discloses a voltage sag reason recognition method based on an EM algorithm and a gradient boosting tree, and aims at solving problems that voltage sags having same kind and different kinds of sag characteristics cannot be classified accurately, data characteristics are hard to extract, and the like. On the basis of voltage sag waveform data and complemented by power grid fault data,thunder data and meteorological data, the per-unit value of power grid secondary voltage is accurately extracted through the EM algorithm, the characteristic of the waveform data in a statistical sense is extracted on the basis of the per-unit value, a gradient boosting tree model is trained in single-phase and three-phase data waveforms through certain training sample data, voltage sag reason recognition is achieved, and a voltage sag caused by a system side fault and a voltage sag caused by a user side heavy user load starting are clearly distinguished.