The invention discloses a three-phase unbalance factor probability assessment method based on state estimation. The method comprises the steps that A, a network node three-phase admittance matrix of an actual electric system is calculated; B, an injection active power expression and a reactive power expression of a phase p of a node i in the electric system are calculated; C, the error between the measured quantity and the true physical quantity of the system is calculated through a state estimation method; D, a sampling process expression of a load power and n distributed power output powers is obtained; E, a state quantity estimated value is solved; F, a voltage unbalance factor is solved; G, a three-phase unbalance factor probability assessment is carried out. By the adoption of the three-phase unbalance degree probability assessment method based on state estimation, three-phase unbalance probability distribution and change characteristics of voltages of a power distribution system which a distributed power supply is connected into can be analyzed objectively and reasonably; the method has positive significance in reasonably utilizing renewable energy to generate electricity in a conventional power distribution system.