LiB3O5 is the most widely used nonlinear optical crystal. Li2Mo3O10 (a nominal composition) is a typical flux used to produce large-sized and high-quality LiB3O5 crystals. The structure of the LiB3O5-Li2Mo3O10 high-temperature solution is essential to understanding the flux behavior of Li2Mo3O10 but still remains unclear. In this work, high-temperature Raman spectroscopy combined with density functional theory (DFT) was applied to study the LiB3O5-Li2Mo3O10 solution structure. Raman spectra of a LiB(3)O(5)Li(4)Mo(5)0(17)-Li2Mo4O13 polycrystalline mixture were recorded at different temperatures until the mixture melted completely. The solution structure was deduced from the spectral changes and verified by DFT calculations. When the mixture began to melt, its molybdate component first changed into the Li2Mo3O13 melt; meanwhile, the complicated molybdate groups existing in the crystalline state transformed into Mo3O102- groups, which are formed by three corner-sharing MoO3 circle divide(-)/MoO2 circle divide(2) (circle divide = bridging oxygen atom) tetrahedra. When LiB3O5 dissolved in the Li2Mo3O10 melt, the crystal structure collapsed into polymeric chains of [B3O4 circle divide(-)(2)](n). Its basic structural unit, the B3O4 circle divide(2) ring, coordinated with the Mo3O102- group to form a MoO3B3O4 circle divide(-)(2) complex and a Mo2O72- group. On the basis of the LiB3O5-Li2Mo3O10 solution structure, we discuss the LiB3O5 crystal growth mechanism and the compositional dependence of the solution viscosity.