The transient response of silicon p-i-n photodiodes irradiated by femtosecond laser with a range of pulse energy levels was investigated experimentally. The spatiotemporal response exhibits three clearly chronological phases. The role of high-injection induced by femtosecond laser effects on the evolving spatiotemporal response was discussed. The results indicate that the space-charge screening effects lead to three clearly separable collection phases with ambipolar diffusion primarily determining the length of the induced transient response signal. Increased femtosecond laser pulse energy levels would further elongate the transient length which would severely degrade the performance of photodetector especially in the high speed signal detection.