U-5.7Nb-xTi (wt. %) alloys with different Ti contents were fabricated by arc-melting. With the increasing of Ti contents, γ0 phase was first to appear in the α″ phase matrix, followed by its propagation. After completely transforming to γ0 phase at U-5.7Nb-1.2Ti, additional Ti alloying led to the formation of the equilibrium γ12 phase. After the Ti atom was equivalently converted to the equilibrium niobium concentration Nbeq, the boundary of each phase composition was very consistent with the U-Nb alloy. Meanwhile, the hardness of U-5.7Nb-xTi alloys were similar with those of U-Nb alloys with same total atomic contents. By analyzing the mechanism of transformation of the U-alloy and the theory of solid solution strengthening of the metal materials, the phase structure and mechanical behavior were considered to be closely related to the yield strength of the parent phase and twinning energies, respectively. Due to their similar physical parameters including atomic radii and bulk modulus, the solution strengthening effects of the parent phase and variation of the twinning energies led by Ti or Nb alloying were similar, which were responsible for the similarities of the phase structure and the mechanical properties in U-Nb-Ti and U-Nb alloys. According to the same physical mechanism, U-Nb-X ternary alloys (or multi-element alloys) can be more accurately predicted the phase structure and the mechanical properties by analyzing physical parameters such as the atomic radius and bulk modulus of alloying element X. These results and analyses provided theoretical basis and guidance for further strengthening design of U-5.7Nb alloy.