Time-resolved reflectivity changes of silicon-on-insulator (SOI) with different crystalline silicon film thicknesses have been measured using the femtosecond (λ=800 nm) pump-probe technique to investigate the ultrafast carrier dynamics. The underlying physics of carrier diffusion and recombination processes is discussed using the reflectivity model, which is based on the time evolution of photoexcited carrier density and temperature. The research shows that, the contribution of surface recombination velocity (SRV) dominates the response of excited carrier dynamics, and the SRV increases with thickness of silicon thin films decreasing. The corresponding active surface state density can reach 1015 cm-2. For relatively small SRV, the transient reflectivity changes are dominated by the ultrafast response of photoexcited carriers. But the contribution of lattice temperature appears to be more significant with large SRVs, which causes the reflectivity to recover and surpass the initial value in shorter time.