(Mg, Fe)SiO3-perovskite is currently considered to be the most abundant mineral in the earth’s lower mantle. Its behavior at high temperature and high pressure is crucial for interpreting conditions at the deep level of the mantle, variations of seismic waves, and so on. Equilibrium crystal structures and mechanics properties of MgSiO3 and (Mg0.75, Fe0.25)SiO3 are determined using first-principles calculations in a series of hydrostatic pressures up to 140 GPa. Seismic wave velocity as a function of pressure is derived from the Voigt-Reuss-Hill scheme. Their thermodynamic quantities under the conditions of the lower mantle’s pressures and temperatures are computed by means of the Debye model within the quasi-harmonic approximation. The substitution effect of Fe2+ on the thermoelastic property for silicate perovskite is discussed. Substitution of Fe2+ for Mg2+ can provoke softening wave velocity phenomenon arising from the minerals containing Mg element located in the earth interior. The present theoretical results are useful for interpreting seismic wave velocity softened in certain areas of the mantle.