In this paper, the femtosecond laser ablation of silicon is investigated by a two-dimensional hydrodynamic model. The ablation depth of the silicon wafer ablated in air at different laser intensities is calculated, and the corresponding experimental measurements are carried out for validation. Two different ablation regimes have been identified by varying the laser fluence. While two-photon absorption dominates in the low fluence regime (<2 J/cm2), electron heat diffusion is a major energy transport mechanism at higher laser fluences (>2 J/cm2). The ablation efficiency first increases with the laser fluence, and reaches the peak value at the laser fluence around 8 J/cm2. It starts to drop when the laser fluence further increases, because of the early plasma absorption of the laser beam energy.