The present application discloses a liquid seal method for on-off of gas pipeline and a device thereof, wherein a liquid-sealed U-shaped container being provided in the gas pipeline that requires to perform on-off control; a lifting mechanism using a leading screw controls the slider having a certain volume to immerse in or leave the liquid so as to raise or lower the liquid level. The U-shaped container has a structure with a wide top and a narrow bottom, the space of the U-shaped container is divided into two spaces with different sizes by a separating plate, and a liftable liquid-occupying device is located at the space having a larger volume; the upper and lower parts of the slider are tapered; and the driving device can be motor or hand wheel.