A15-type Nb₃Sn, owing to its high superconducting transition temperature, high upper critical field, and potential for superconducting radio-frequency applications, is regarded as a promising candidate for next-generation superconducting radio-frequency functional films. In this study, Nb₃Sn thin films were prepared on single-crystal Nb substrates by Nb/Sn dual-target magnetron co-sputtering followed by vacuum annealing at 900 °C, with the aim of clarifying the microstructural evolution and growth-controlling mechanism during sputtering and heat treatment. The growth of Nb₃Sn films is not governed solely by the Nb substrate orientation or by simple interfacial epitaxy, but by the coupled effects of Nb substrate orientation, Sn diffusion, local compositional fluctuation, and interfacial defects. The Nb substrate orientation can locally induce the orientation relationship of (210)_Nb3Sn//(110)_Nb and contribute to interfacial strain relaxation; however, because the film is composed of competing multi-oriented grains, its role is mainly local and modulatory. By contrast, dislocations, grain boundaries, and interfacial defects provide preferential short-circuit diffusion paths for Sn and therefore dominate local Sn enrichment, A15 phase nucleation/growth, and the evolution of compositional inhomogeneity. After heat treatment, the superconducting transition temperature (T_c) increases from 9.8 K to 12.6 K, indicating the promoted formation of the A15-Nb₃Sn phase, whereas the broadening of the transition width (ΔT_c) to 6.0 K suggests that residual compositional and structural inhomogeneities still limit further performance improvement.