Abstract: A series of experiments were performed on Yang accelerator and PTS facility. We measured velocity histories of electrode panel with/without single crystal ［100］LiF windows by Doppler pins system(DPS, laser wavelength 1550 nm). As the driven panel is not disturbed until the first step of free-surface velocity profile, it is feasible to utilize half of free-surface velocity to estimate sample-window interface velocity. Analyzing apparent velocity profiles of the electrode panel with windows and free-surface, we obtained velocity correction and change of refractive index under magnetically driven quasi-isentropic compression to 50 GPa. Supposing half of free-surface velocity as true velocity, and through linear fit, we obtained the velocity correction factor. Additionally, the relationship between refractive index and density was calculated in the range of experimental conditions by a new procedure. The experiment data measured by photon Doppler velocimetry under 1550 nm laser wavelength are in close agreement with results from D.E. Fratanduono, Y. Ma, B.J. Jensen. The measured uncertainty in this work is as good as several shock experiments.