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The Nb 3 (Al 1-x Sn x) superconductors (x = 0.05, 0.15, 0.25, 0.35) were prepared by mechanical alloying and subsequent low-temperature sintering. It was found that the critical current density (J c) of the obtained Nb 3 (Al 1-x Sn x) sample is enhanced significantly compared to undoped Nb 3 Al prepared using the same method. The maximum value of the J c reaches up to 5.10 × 104 A·cm−2 (4.2 K, 5 T) for Nb 3 (Al 0.85 Sn 0.15) sample. Combined with the phase composition analysis and microstructure observation, it is indicated that Sn enters the A15 crystal structure of Nb 3 Al forming the Nb 3 (Al 1-x Sn x) phase after low-temperature sintering. What's more, Sn doping improves the grain connectivity as well as refines the grain size of Nb 3 Al, which are the main reasons for the enhancement of J c in the Sn doped samples. Based on the exploration of the phase evolution of Nb–Al–Sn system during the mechanical alloying and subsequent low-temperature sintering process, it is also confirmed that Nb 3 Sn forms firstly during the sintering process and then can serve as the perfect nucleus of Nb 3 Al to increase its nucleation rate, leading to the significant decrease of the grain size of Nb 3 Al. • The J c of Nb 3 Al reaches up to 5.10×104 A·cm−2 (4.2 K, 5 T) after Sn doping. • The Sn doping sharply reduced the grain size of Nb 3 Al (from 80-200 nm to 25–40 nm). • During the low-temperature sintering process, the Nb 3 Sn formed firstly and then served as the perfect nucleus of Nb 3 Al to increase its nucleation rate. [ABSTRACT FROM AUTHOR]