The tire low-frequency handling model and the high-frequency durability model were unified, and the UniTire tire model including in-plane dynamic characteristics was proposed. The steady-state UniTire is the zero-frequency output of the tire dynamic system. When the frequency of the tire motion input signal is infinite, the output of the wheel center force is zero. Using these as boundary condition, the transfer characteristics of the tire dynamic system are established. Considering that in the simulation of the vehicle virtual proving ground (VPG), the load history of the wheel center is mainly affected by the rigid ring modes of the tire, that is, the in-plane frequency is less than 100 Hz, and the out-of-plane frequency is less than 60 Hz. Higher frequency components contribute to the load analysis of the wheel center indistinctively, so a second-order transfer model that satisfies the boundary conditions is established. Finally, using typical tire dynamic test data, the tire model parameter identification and test verification are carried out. The comparison results show that the proposed model can accurately express the in-plane tire dynamic characteristics.