The first-principles calculations based on density functional theory with projector-augmented wave are used to study the physical properties of monoclinic MgCO3 at lower mantle conditions. The results show that the phase transition pressure of rhombohedral MgCO3(magnesite) to monoclinic MgCO3 is between 72–79 GPa in the temperature range of the mantle. The equation of state for monoclinic MgCO3 agrees well with recent experimental results. The elastic constants of monoclinic MgCO3 are consistent with the latest theoretical results. The elastic modulus of monoclinic MgCO3, especially its shear modulus, exhibits a nonlinear pressure dependence, leading to a significant nonlinear pressure dependence of the small elastic anisotropy and the small wave velocity anisotropy, which is why carbide minerals may not be detected in the deep mantle. The thermodynamic properties of monoclinic MgCO3 at high temperature and high pressure are predicted using the Debye-Grüneisen model.