Three new alkali metal molybdate crystals LiRbMo₃O₁₀, LiCsMo₃O₁₀ and KCsMo₃O₁₀ have been synthesized, and their structures have been determined by single-crystal X-ray diffraction. LiRbMo₃O₁₀ and LiCsMo₃O₁₀ are isostructural and crystallize in the orthorhombic space group Pnma, whereas KCsMo₃O₁₀ belongs to the space group P. Moreover, we have also obtained NaCsMo₃O₁₀ crystal, and its unit cell dimensions have been previously reported. Crystallographic analysis reveals that NaCsMo₃O₁₀ crystallizes in the space group Pnma. The effect of cations size on their framework structures has been discussed in detail. In addition, infrared spectra are used to verify their structural validity. Furthermore, the UV-Vis-NIR diffuse reflectance spectra are also presented. Finally, the first-principle theoretical studies are carried out to increase the understanding of electronic structures and linear optical properties.