The knowledge of phase transition of material under dynamic loading is an important area of research in inertial confinement fusion and material science. Though the shock-induced phase transitions of various materials over a broad pressure range have become a field of study for decades, the loading strain rates in most of these experiments is not more than 10^6\;\rms^ - 1 . However, in contrast with the strain rate range where the phase diagram is a good predictor of the crystal structure of a material, at higher strain rate ( > 10^6\;\rms^ - 1 ) the phase diagram measured can be quite different not only in shifting the boundary line between various phases, but also in giving a different sequence of crystal structure. High-power laser facility can drive shock wave and simultaneously provide a precisely synchronized ultra-short and ultra-intense X-ray source. Here, based on the Prototype laser facility, an in situ X-ray diffraction platform for diagnosing shock-induced phase transition of polycrystalline material is established. The in situ observation of material phase transition under high-strain-rate shock loading is carried out with typical metals of vanadium and iron. Diffraction results are consistent with vanadium remaining in the body-centered-cubic structure up to 69 GPa, while iron transforms from the body-centered-cubic structure into hexagonal-close-packed structure at 159 GPa. The compressive properties of vanadium and iron obtained in in situ X-ray diffraction experiment are in good agreement with their macroscopic Hugonoit curves. The decrease in the lattice volume over the pressure step period yields a strain rate on the order of 10^8 - 10^9\;\rms^ - 1 . The available of the presented in situ X-ray diffraction plateform offers the potential to extend our understanding of the kinetics of phase transition in polycrystalline under high-pressure high-strain-rate shock compression.