On the SILEX-Ⅰlaser facility, we measured the transmission spectra after ultrashort and ultraintense laser pulse interaction with tenuous plasma. Due to the existence of the wakefield, the time-dependent refractive index of the plasma can accelerate or decelerate photons of laser pulse when the laser pulse propagates in the plasma. Its main features are the spliting of the fundamental frequency and its evident broadening in the laser transmission spectra.The side_band wave which is often associated with Raman and Self modulation were not observed in our experiments. Utilizing the LPIC++ code, we also simulated the transmission spectrum after the ultrashort_ultrahigh intensity laser pulse interacts with the tenuous plasma. The simulation results are in good agreement with the experiment results.