We investigate the impact of pulsed excimer laser anneal on fully strained germanium-tin alloys (Ge1-xSnx) epitaxially grown on Ge substrate by molecular beam epitaxy. Using atomic force microscopy, X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy, the morphological and compositional evolution of Ge1-xSnx with Sn content up to 17% after annealing using various conditions is studied. Ge0.83Sn0.17 samples annealed at 80 mJ/cm2 or 150 mJ/cm2 have no observable changes with respect to the as-grown sample. However, Ge0.83Sn0.17 samples annealed at 250 mJ/cm2 or 300 mJ/cm2 have Sn-rich islands on the surface, which is due to Sn segregation in the compressively strained epitaxial film. For Ge0.89Sn0.11, significant Sn redistribution occurs only when annealed at 300 mJ/cm2, indicating that it has better thermal stability than Ge0.83Sn0.17. A mechanism is proposed to explain the formation of Sn-rich islands and Sn-depleted regions.