In order to systematically understand the mechanism of martensitic transformation of NiTi shape-memory alloy, and to study the effect of temperature and stress on the electronic structure, we have carried out first-principles calculations based on the density-functional theory (DFT). Analysis of density of states shows that martensitic transformation induces the Fermi face to shift and the density of states near the Fermi face to decrease. With the temperature decreasing and deformation increasing,the density of state of B2 phase and the degree of energy band overlaps increase，thus the stability decrease.