TC21 alloy was carried out on Gleeble–3800 thermal compression simulation tester at temperatures and strain rates ranging of 830–1010℃ and 0.0005–10s^–1. Based on the compressed experimental data, taking the hot deformation constitutive relation model derivation of Arrhenius and the hot processing map construction under different instability criterion as the main line of the study, the constitutive model under different hot deformation parameters and the hot processing map under different instability criterion (Gegel, Malas, Prasad, Murty, Semiatin) were obtained. Then, the theoretical basis and prediction results of above five instability criterion were analyzed. By using the theory of hot processing map combining with microstructure, the suitable forming and flowing instability areas were predicted, and hot working processing parameters of TC21 alloy are optimized scientifically and rationally.