The water bags are the decelerator of rocket sled high-speed test, which added mass is large, and the operating speed is fast, thus having to bear great load when using the impact of water to decelerate. In order to ensure the safety of the water bags structure and achieve the expected deceleration effect, numerical simulation analysis must firstly be done on the water bags movement condition. Using the fluid-solid coupling technology of Abaqus and the method of CEL, the movement condition of the water bags is analyzed. Using the resistance curve obtained from the simulation analysis, and based on fluid resistance theoretical calculation formula, the calculation formula of the water bags resistance is acquired. The water bags resistance under different speed and initial water level can be calculated from this calculation formula, thus obtaining the deceleration performance of the water bags.