An inner detecting device for subsea oil and gas pipeline includes a support shaft defining a sealed chamber therein; a packer cup coupled to the support shaft; a hinge assembly coupled to the support shaft, swinging with respect to the support shaft in a radial direction and sliding with respect to the support shaft in an axial direction; a magnetic detector connected to the hinge assembly to move with respect to the support shaft in the radial and axial directions, and the magnetic detector tending to move away from the support shaft in the radial direction; a battery pack; an electric recorder electrically connected to the battery pack; and a mileage recorder recording a travel distance of the inner detecting device within the pipeline.