TiAl alloy represents a new class of light and heat-resistant materials. In this study, the effect of temperature, pressure, and grain size on the high-temperature creep properties of nanocrystalline TiAl alloy have been studied through the molecular dynamics method. Based on this, the deformation mechanism of the different creep stages, including crystal structure, dislocation, and diffusion, has been explored. It is observed that the high-temperature creep performance of nanocrystalline TiAl alloy is significantly affected by temperature and stress. The higher is the temperature and stress, the greater the TiAl alloy’s steady-state creep rate and the faster the rapid creep stage. Smaller grain size accelerates the creep process due to the large volume fraction of the grain boundary. In the steady-state deformation stage, two kinds of creep mechanisms are manly noted, i.e., dislocation motion and grain boundary diffusion. At the same temperature, the creep mechanism is dominated by the dislocation motion in a high-stress field, and the creep mechanism is dominated by the diffusion creep in the low-stress field. However, it is observed to be mainly controlled by the grain boundary diffusion and lattice diffusion in the rapid creep stage.