Thermal annealing treatments were performed to improve the crystal quality of Ge_0.946Sn_0.054 epilayers grown on Si(100) substrates by molecular beam epitaxy (MBE). When the annealing temperature was lower than 500°C, hardly any change was observed. When the annealing temperature was too high, the Sn segregation occurred, resulting in holes in the surface and a reduction in the epilayer Sn content. Annealing treatment at 500°C for 30 s is suitable for improving the quality of the Ge_0.946Sn_0.054 alloy epilayers. This study presents a reasonable method to improve the crystal quality of GeSn alloys and the performance of related devices.