In this paper, the lattice parameter and bulk modulus of lithium(Li) at different pressures and temperatures are calculated by using the density functional theory method within the generalized gradient approximation(GGA).Through the quasi-harmonic Debye model, the thermodynamic properties of Li are predicted. The dependences of the normalized primitive cell volume V/V0 on pressure P, the variation of the thermal expansion coefficient α with pressure P and temperature T, as well as the dependences of the heat capacity Cv on pressure P and temperature T are obtained systematically in the ranges of 0~100 GPa and 0~2000 K.