The quasiparticle and longitudinal acoustic (LA) coherent phonon dynamics of the FeSe0.9S0.1 single crystal were investigated by femtosecond transient optical spectroscopy. The FeSe0.9S0.1 single crystal undergoes metal-superconductor transition at Tc = 9.5 K and structural phase transition at Ts = 72 K. To investigate the correlation between the quasiparticle/LA coherent phonon dynamics and the antiferromagnetic spin/nematic fluctuation, we measured the transient differential reflectivity ΔR(t)/R0 in time series (R is the reflectivity) between 4-130 K. The ΔR(t)/R0 time series showed a negative sign below Ts = 72 K, while its sign reversed from negative to positive above Ts. The ΔR(t)/R0 time series was contributed by a combination of two exponential decays and an oscillation term. The slow decay attributed to electron-phonon and phonon-phonon coupling showed a negative amplitude below Ts and a positive amplitude above Ts. A fast decay with positive amplitude appeared at T ≥ 55 K was induced by the electron-phonon coupling of the high-temperature orthorhombic phase FeSe0.9S0.1. The oscillation contributed by the LA coherent phonon emerged below Ts but was not observed above Ts, indicating that it was impacted by the nematic fluctuation of FeSe0.9S0.1.