In order to enhance the precision of springback simulation for transformation-induced plasticity steels, a combined hardening model is presented on the basis of Voce nonlinear isotropic hardening and Chaboche kinematic hardening theories. A finite element model of uniaxial tension subjected to orthogonal strain path is built using LSDYNA software. Minimizing the deviation between the predicted stress-strain curve and the experimental one is regarded as the object, the parameters of the hardening model are determined by combining using LS-OPT software. This hardening model is introduced to simulate the springback of U-shaped part that is made of TRIP780 sheet steel. Experimental verification is conducted. The error factors attributed to the simulation and experiments are summarized in the end. The results show that the combined hardening model based on the Voce and the Chaboche hardening model with two terms of backstress is of higher precision in the springback simulation than the isotropic hardening model has. The selection of the approximate models used in the optimization, prestraining levels and U-shaped parts profile measurement are the important error sources. The method of combing optimization package and finite element software to determine the parameters in hardening model is of high precision and feasibility in practice.