An improved scheme of optical pump terahertz probe (OPTP) technique was proposed. Using an ultrashort broadband terahertz (THz) pulse of air-induced plasma radiation ionized by a two-color femtosecond laser field as the probe pulse, the photo-excited dynamics in silicon was characterized with the improved OPTP, and the capability of the OPTP system based on the improved scheme of this paper was verified through a characterization process. The pump depth of silicon increase with higher pump power, so that the pump depth of 400nm pump pulse is greater than that of 800nm pump pulse. The pump depth of p-doped silicon is the largest, followed by intrinsic silicon, and the smallest for n-doped silicon while using 400nm pump pulse. However, the pump depth of p-doped silicon is the largest, followed by n-doped silicon, and the smallest for intrinsic silicon while using 800nm pump pulse. Furthermore, based on the ultrawide bandwidth and ultrashort pulse width of the probe pulse, the sub-picosecond phonon wavepacket oscillation in excited state is observed.