The invention belongs to the technical field of well completion of petroleum and natural gas industries, and particularly relates to an electro-hydraulic downhole casing hole opener. The electro-hydraulic downhole casing hole opener is characterized by comprising a ground system, an electro-hydraulic power unit connected to the ground system through a cable and a hole opening unit connected to theelectro-hydraulic power unit; the electro-hydraulic power unit comprises a support frame, a fuel tank, a motor, a hydraulic pump, a relief valve connected to the hydraulic pump, an electromagnetic directional valve I and an electromagnetic directional valve II, wherein the electromagnetic directional valve I and the electromagnetic directional valve II are connected to the output end of the relief valve and arranged in parallel; and the hole opening unit comprises a unit body provided with four flow passages of the flow passage A, the flow passage B, the flow passage C and the flow passage D,a hydraulic motor, a piston bush connected with a motor output shaft through a spline, a drill bit in threaded connection with the piston bush and a pressing cap. The electro-hydraulic downhole casing hole opener transmits electric energy to the downhole from the ground through a cable, and the downhole hole opener is provided with the motor and a hydraulic power system, drives the drill bit to rotate and cut a casing, realizes hole opening of the casing, and provides conditions for implementing a next step of operation.