In this paper,the vehicle stability is analyzed on the basis of phase plane method,and a vehicle stability criterion is determined according to the change laws of the phase plane.First,a nonlinear single- track vehicle mo- del is constructed by using vehicle test data.On this basis,a sideslip angle- sideslip angular velocity phase plane is established as the stability criterion.Then,by analyzing the shortcomings of the existing phase stability region divi- sion methods,a new method using five characteristic values to determine the diamond stability region is proposed,specifically,the diamond region moving with the equilibrium point is selected as the stability region according to the change laws of the phase plane.Moreover,a dataset of the characteristic values of the stability region under full condition is built by a large number of simulations and the change laws of both the characteristic values and the sta- bility region are discussed.Finally,the results obtained by the stability criterion based on phase plane method are used to determine the introduction of the stability algorithm.Simulation results show that the proposed stability cri- terion can provide a basis for stability control algorithms and improve vehicle stability.