The chaotic motion of 1/4 vehicle suspension model with fractional nonlinear characteristic under dual frequency excitation is studied. Using the Melnikov method，the analytical necessary conditions for heteroclinic chaotic motion of the system are derived，and the threshold of the chaotic boundary surface of the system is obtained. The influence of various parameters of the suspension system on the chaotic boundary surface is discussed. Time history diagram，spectrum diagram，phase diagram，Poincaré cross section diagram and maximum Lyapunov index are used for numerical verification. The results show that there is chaotic motion in the suspension system under dual frequency excitation，and the damping coefficient，stiffness coefficient and other parameters have certain influence on the threshold value of the chaotic boundary surface in the suspension system with fractional order，in which the fractional order term and coefficient and linear damping coefficient have great influence.