In a tungsten-based alloy system, the appropriate solute elements are selected to produce strong segregation effect to reduce the interfacial formation energy, which can effectively improve the mechanical property and thermal stability of the system. Based on the first principles calculation, the solute segregation model of tungsten-based alloys is constructed. The W-In alloy is taken for example to study the grain boundary segregation behavior and bonding characteristics of solute at different concentrations. The bonding of the W-In system is revealed from the electronic structure, and the variation of the interface stability of the W-In system with the solute concentration is predicted. Based on the electronic structure analysis of bond population, differential charge density and density of states, the bond transition characteristics of solute atoms in the W-In system in the segregation process are found, and the microscopic mechanism of the W-In bond transitioning from the ionic bond inside the grain to the strong covalent bond in the grain boundary region is elucidated: the difference between the grain boundary and the intragranular structure leads to a decrease in the valence state of the W atom in the grain boundary and the oxidizability is weakened, eventually leading to the W-In bond transition. The non-monotonic variation of the intrinsic segregation energy of the solute with the concentration of In in the W-In system is obtained. The mechanism of the influence of solute concentration on the intrinsic segregation energy is revealed by analyzing the bond interaction and energy: the solute concentration remarkably affects the bond strength before and after the W-In bond segregation, resulting in a significant decrease in the segregation ability when the solute concentration is close to 0.0976, and finally the variation of the segregation energy with solute concentration is obtained. Based on the analysis of the phase mechanical stability and the solute segregation in the grain boundary, without considering the vacancy concentration, the optimal solute concentration range and the range that needs to be circumvented in the W-In alloy system with high thermal stability are predicted by the calculations of the model, which are 0.106−0.125 and 0.0632−0.106, respectively. This study provides theoretical basis and quantitative guidance for designing and preparing the tungsten-based alloy materials with high thermal stability.