The invention relates to a voltage sag evaluation method for a three-phase asymmetrical power distribution network. The method comprises the following steps of: establishing a fault random model of the three-phase asymmetrical power distribution network; establishing a DG (Distributed Generation) random model of the three-phase asymmetrical power distribution network; carrying out orthogonal transformation on the fault random model, and acquiring an uncorrelated fault random model corresponding to the fault random model; processing a random variable consisting of the uncorrelated fault random model and the DG random model by adopting a point estimation method, and obtaining random variables of two sets of simulation schemes; carrying out reverse decomposition on the random variables of two sets of simulation schemes, and obtaining random variables of two sets of related simulation schemes, which correspond to the random variables of the two sets of simulation schemes; by a Cornish-Fisher series, respectively determining probability density functions of elements in the random variables of two sets of related simulation schemes; and comprehensively considering correlation of a fault type and a fault line in the power distribution network and various short circuit faults in the power distribution network to simulate characteristics of the integral power distribution network so as to provide reference for adopting measures of inhibiting a voltage sag.