Ternary lithium metal halides have attracted much attention as potential solid electrolytes. In this work, we study the structural, electronic and ionic diffusion properties of a series of Li_xYCl_3+x (x = 2.14, 3.00, 4.20) and Li_xYBr_3+x (x = 1.8, 3.0, 5.0) by using first-principles calculation based on density functional theory. The calculation results show that the Li-ion concentration has a significant effect on the properties of the materials, and with the increase of x value, Li-ion number becomes higher, structure turns more stable, band gap gets larger, and migration barrier lowers, thus the performance of the material can be tuned. In addition, the calculation results further show that Li₃YCl₆ and Li₃YBr₆ with the best balance between Li-ion carrier concentration and vacancy concentration exhibit the highest structural stability, the largest band gaps, and the lowest migration barriers in all similar structures. Our study provides a new strategy and idea for designing better-performance halide solid electrolytes.