The crystal structures, electronic structures, atomic populations, charge density distributions and birefringence of Li2MoO4 , Na2MoO4 , K2MoO4 and Rb2MoO4 were compared by first principles. The results show that the bandgaps of the four crystals are direct band gaps, the differences of the bandgap between them are not large, and they are all around 4. 3 eV. As the radius of alkali metal atoms increases, the contribution of their electronic orbitals to the energy bands moves closer to the high-energy side. The distribution of the bonding boule number and electron density suggests that covalent bonds are formed between Mo and O, and the alkali metal atoms form ionic bonds with O. Rb2 MoO4 has the highest birefringence of 0. 022 5 among the four crystals at 1 064 nm. The refractive indices of the four crystals show that the radius is positively correlated with the birefringence. The study in this paper provides some references for the application of molybdate in nonlinear optical crystals.