The key to using the particle flow strength reduction to evaluate the slope stability is the judgment of the critical failure state. Tooking a slope project as an example, the PFC2D code was used to simulate a biaxial compression test on the soil to calibrate its mircroscopic parameters. A particle flow model of the slope was established. Then, the criterion of displacement mutation was improved. The stability factor of the slope was solved by drawing the relation curve between characteristic particles and reduction coefficients. Besides, the selection of the characteristic particles was studied. The method in this paper can reproduce the morphological changes of slope sliding failure. The simulated failure mode of slope is consistent with the finite element strength reduction method and the Bishop method. The research results can provide references for the particle flow strength reduction to solve the stability factor of slope and the simulation of landslides.