The invention discloses an oil cylinder structure of a hydraulic jack, which belongs to the technical field of lifting tools. The oil cylinder structure comprises a jacket, an oil cylinder, a hood and a piston, wherein the oil cylinder is arranged in the jacket, and an oil storage cavity is formed between the outer wall of the oil cylinder and the inner wall of the jacket, the hood is in fit connection with the upper part of the jacket as well as the upper part of the oil cylinder, the piston is arranged in a cylinder cavity of the oil cylinder, the lower part of the piston is provided with a piston ring and a piston seal, and the upper part of the piston is corresponding to and matched with a piston hole of the hood. The oil cylinder structure is characterized in that a part on the inner wall of the upper part of the oil cylinder and corresponding to a position that the piston arrives at the ascending limit is provided with at least a drain pan, a part on the inner wall of the hood and in fit connection with the upper part of the oil cylinder is provided with a longitudinal hood drain pan, and the hood drain pan is communicated with the oil storage cavity and the upper cavity of the oil cylinder. The oil cylinder structure disclosed by the invention has the advantages that situations that the hood is fractured and the oil cylinder is deformed or out-of-service can be eliminated so as to ensure safety.