The pressure effects on the elastic properties of lithium doped by hydrogen are studied using first-principles methods based on density functional theory. A supercell including 16 Li atoms in hcp lattice is constructed with a hydrogen atom interstitially doped in. The supercell is fully relaxed at 0K. The elastic properties of the stable lattice are studied. The result shows that the bulk modulus increases after hydrogen atom is added. The elastic constants C11, C33, C66 and C12 are larger than those of pure Li，while the shear modulus C44 is smaller. The elastic constants increase with external pressure, with C11, C33 and C66 larger than those of pure Li while C12 being smaller. C13 is abnormally lower than that of pure Li within the pressure range from 2GPa to 4GPa, while C44 and C13 are greater at higher pressure area. The discrepancy of shear modulus between the doped and the pure system is enhanced by external pressure. The analysis of anisotropy ΔW shows that the doped system maintains isotropic under external pressure as does pure Li.