Abstract: The hot deformation behavior of 20CrMnTi alloy steel was studied by using a Gleeble-1500D thermal simulation testing machine, with deformation temperatures ranging from 900 ℃ to 1200 ℃ and strain rates ranging from 0.01 s^-1 to 10 s^-1. The rheological stress changes under different deformation conditions were analyzed. The Arrhenius constitutive equation of 20CrMnTi alloy steel was obtained through linear fitting based on experimental data. Then, the multi-directional controllable forming process of the 20CrMnTi rack piston with φ120 mm cylinder diameter was simulated using numerical simulation software, and finally, the process parameters obtained from the simulation were verified through process experiments. The results indicate that the modified constitutive equation can accurately predict the hot deformation behavior of 20CrMnTi alloy steel. The φ120 mm cylinder diameter rack piston forging obtained by the process test is fully filled, which is basically consistent with the simulation results.