The invention provides a cubic oil cylinder and a sealing ring and a sealing structure for the cubic oil cylinder. The cubic oil cylinder comprises a cylinder block with a square section, a piston, apiston rod, and the sealing structure arranged between the piston and the cylinder block; the sealing structure comprises a first sealing bag ring and a second sealing bag ring arranged at the upper and lower parts of the piston, and a third sealing bag ring arranged in a cylinder block opening; annular sealing grooves are correspondingly formed in the outer wall of the piston and the cylinder block opening; the sealing bag rings are embedded in the corresponding annular sealing grooves, and are contacted with the inner walls of the grooves; the inner groove surfaces of all the sealing groovescommunicate with the cylinder block of the oil cylinder; and when the pressure injection is performed on hydraulic oil, the hydraulic oil is injected in the inner groove surfaces of the sealing grooves to apply pressure to the corresponding sealing bag rings for sealing of the piston and the cylinder block. The cubic oil cylinder is reliable in sealing mode, solves the problems of easy damage caused by nonuniform contact pressure of the sealing piece and sealing of the cubic oil cylinder, and can be used for replacing a cylindrical oil cylinder in a specific place.