Submarine power cable (SPC) is laid in a complicated circumstance, bearing multiple stress of electro-thermal-mechanical field. Many kinds of emergency may occur during cable operation. The aim of this study was to figure out the impact of moisture infiltration on a 500 kV AC oil-filled SPC after accidental external striking. Different from XLPE cable, it is of high possibility that moisture will infiltrate and diffuse into oil-filled cable because of its oil paper insulation. To analyze the characteristics of electric-thermal field inside the cable, a three-dimensional 500 kV AC oil-filled SPC model was established and a mushroom-shaped region of oil paper with high moisture content was inserted into insulation layer. The model was analyzed in different sizes of moisture infiltration region using finite element method (FEM). The results show that the maximum temperature rose to 153.57 ℃ and the maximum electric field was up to 57.03 kV/mm at the worst case, which are harmful to the operation of the whole cable. In addition, the relation between electric-thermal field and moisture region parameters were given.