TiAl－based alloys containing high content of Nb are significantly promising for high-temperature structural applications in aerospace and automotive industries, due to their low density, excellent high temperature strength, high resistance to oxidization and creep resistance. However, poor hot workability limits their extensive applications. Owing to sufficient number of independent slip system, small deformation resistance, apt to plastic forming of disordered bccβ phase at elevated temperature, the novel β－γ TiAl with high content of Nb alloys exhibit excellent hot deformability. The quasi isothermal forging process of Ti－45Al－8.5Nb－0.2W－0.2B－0.02Y alloy is investigated by using a Deform－3D software. The effective stress, effective strain, temperature distribution of the alloy are analyzed. In order to obtain the critical damage factor of the alloy under the condition of 1150℃ and strain rate 5×10^-2s^-1, the hot compression physical simulation experiment of the alloy is performed to achieve true stress－strain data. By inputting these data into the Deform－3D software to simulate the true compression process, the critical damage factor is obtained. The results demonstrate that with increase of strain rate, the temperature loss becomes less obvious, the effective stress and the maximum effective strain rises up, the deformation uniformity ratio decreases and the deformation becomes more uniform. Combined with the simulation and experiment results, the critical damage factor of the β－γ TiAl alloy containing high content of Nb is 0.206 during the quasi isothermal forging process at\linebreak 1150℃ and strain rate 5×10^-2s^-1.