For the problem of curled skin defect on the surface of 9Ni steel hot-rolled plate, its micro characteristics of curled skin were systematically studied by scanning electron microscopy, energy dispersive spectroscopy, etc., and the causes of curled skin defects on the surface of 9Ni steel were investigated by thermal simulation testing machine Gleeble-3800 and thermogravimetric analyzer. The defect analysis results show that there are obvious oxide layers on both sides of crack inside the curled skin defect, and there are numerous grain boundary network cracks at the end of crack and its extension line. The mechanism research shows that during the high-temperature oxidation process of 9Ni steel billets, the oxides of Ni and Fe intertwine to form a rich Ni particle layer, and the co-segregation of Ni with Si, Mn and other elements forms oxide dots and a network structure of grain boundaries, which extends to the surface of steel plate during the hot rolling process, leading to the occurrence of curled skin defects. Thus, by controlling the temperature of heating furnace for 9Ni steel, increasing the descaling pressure, and adjusting the distribution of reduction rate in the rolling process, the surface curled skin defects of 9Ni steel can be effectively reduced.