In a method of blocking a water-out zone in a gas well by dumping cement and injecting pressurizing gas, a gas is injected into a target well to push and press accumulated water in the well back into the formation surrounding the well, so that the well is water-free; meanwhile, a dump bailer is lowered to the water-out zone in the well using a wire line. When the dump bailer has reached the water-out zone, it is controlled to open a bottom opening thereof to release cement milk loaded therein. The released cement milk accumulates in the water-out zone and then slowly flows into a gravel layer between a screen pipe and a wall of the well. The gas injection continues until the cement milk is cured and hardened to achieve the object of blocking the water-out zone.