The shut-off valve testing system provides for the testing of the main shut-off valve of a combustible gas supply line in such facilities as refineries, factories, or other plants utilizing such gaseous fuel. The system includes a double-ended hydraulic cylinder receiving hydraulic pressure from a suitable source, the cylinder communicating hydraulically with a hydraulic actuator for the main shut-off valve. The system provides for testing of the shut-off valve by actuating the valve through a portion of its full travel, thus confirming that the valve is free. This is accomplished by shutting off the pressure to one side of the double-ended hydraulic cylinder, and opening the hydraulic line between the cylinder and the actuator. Thus, hydraulic pressure from the actuator can bleed to the cylinder, allowing the actuator to move to the extent of the limiting spring and/or hydraulic pressure to the opposite side of the cylinder.