Abstract: In the numerical simulation of hypersonic chemical nonequilibrium flows, species density may become negative at the beginning of calculation, chemical stiffness is the main reason that affects the calculation’s stability and convergence. Based on the basic theory of source term’s linearazition, two linearazition methods were chosen. One is based on the physical rules of chemical reaction flows. The method’s advantage is that can improve the calculation’s stability and can avoid the species’ density to become negative during the calculation. The other method is the time preconditioner method, which has the function of diminishing the stiffness of the source term. The method was proved to be well approximates the partial differential equations. The calculation result shows that this method can effectively avoid negative density’s appearance and accelerate the calculation’s convergence. So the method effectively improves the calculation’s efficiency.