Time-resolved linearly polarized pump-probe spectroscopy is used to investigate carrier relaxation dynamics in instrinsic GaAs. Absorption saturation and absorption enhancement are observed. It is found that the absorption saturation can be observed obviously when the photon energy is smaller than 1.549eV, otherwise, the absorption enhancement can be observed at a carrier density of 2×1017 cm-3. When the carrier density is above 7×1016 cm-3, the absorption enhancement increases rather than decreases with delay time. The simulation results with consideration of the competition between band filling effect and band-gap renormalization effect are in good agreement with our experimental results. With the band filling effect and band-gap renormalization effect considered, we develop a new analytical model to retrieve the carrier lifetime.