The microstructure evolution responsible for significant change of mechanical properties during low-temperature aging of U-Nb alloys was investigated by phase-field method. Results show that spinodal decomposition may occur at the early stage only when its thermodynamic condition is satisfied. Possibilities of general precipitation widely exist at low-temperature region, where the Nb content difference between the stable nucleus and the matrix can be significantly small, no matter the matrix of aging is cubic-like phase or in the orthorhombic-like phase, because the alloy composition is close to the critical composition of structural transition. At low diffusion rate and the relatively high mobility of phase interface, the Nb content of growing precipitates is far away from equilibrium composition. It is suggested that the Nb redistribution caused by phase decomposition may be responsible for the low-temperature aging hardening effect in U-Nb alloys.