The invention relates to a pressure maintaining cylinder of a rope coring tool. The cylinder is characterized in that an inner cylinder combination is composed through connection of an inner cylinder upper connector, an inner cylinder, a shell body and a valve base, and a valve core is disposed at the joint of the shell body and the valve base; a hollow core barrel combination is composed through connection of a compensating cylinder upper connector, a compensating cylinder, a limiting connector and a core barrel, the core barrel combination is sleeved inside the inner cylinder combination as a whole and constitutes axial slide seal fitting between the compensating cylinder and the inner cylinder upper connector by a slide sleeve, and a locking mechanism is disposed at the joint of the slide sleeve and the inner cylinder upper connector; and the upper end of the core barrel combination is connected and sealed by the compensating cylinder upper connector, and the lower end of the core barrel combination constitutes spaced seal fitting with the valve core and the valve base. According to the invention, the overall structural design is compact, a locking seal mechanism has a clever structure, and functions including core barrel limiting, core barrel sealing and shifting-out of the core barrel under pressure could be achieved; a core diameter which is as large as possible can be acquired in limited space; safety intensity of the inner cylinder upper connector with a thin wall thickness could be satisfied; and anti-shear strength and anti-extrusion strength of the locking seal mechanism are high.