A pressure cylinder includes a pressure-boosting member set, an actuation member set, and a cylinder body, which defines two transversely extending air inlets, two transversely extending air outlets and two air passages respectively axially connected between the two air inlets and the two air outlets for allowing an applied compressed gas to flow through the two air inlets and the two air passages and the two air outlets into an air chamber in the cylinder body to move the pressure-boosting member set and the actuation member set in an oil accumulation chamber in the cylinder body. The hidden loop design for guiding the applied compressed gas does not require any extra pipelines and connectors, avoiding pipeline deterioration or connector loosening problems.