As a pyrolysis medium for reservoir, supercritical water can dissolve all kinds of organic matter, promote the transformation of organic matter, reduce semi-coke yield and improve the pyrolysis efficiency. In this study, the supercritical water pyrolysis experiment of Chang 7 medium-low maturity shale was carried out through the self-constructed supercritical water high-temperature and high-pressure pyrolysis experimental device. The evolution of pore structure of shale after supercritical water is systematically analyzed by N2 adsorption/desorption. The results show that after supercritical water treatment, the specific surface area, pore volume and average pore diameter of shale increase significantly. Supercritical water as a pyrolysis medium is conducive to the development of shale pores. After supercritical water treatment, a large number of unconverted kerogen and undischarged heavy hydrocarbon in the core are cracked into oil and gas products, and escape from the core matrix to form a large number of organic pores. The expansion of pores inside shale and the development of effective pores are conducive to the formation of more seepage channels inside shale, thus increasing the connectivity of shale matrix. The above research results provide an important theoretical basis for the application of in-situ extraction technology of shale supercritical water.