Since the thrust characteristics directly affect the launch speed, altitude and flight range of the water rocket, in order to improve the thrust performance of the water rocket, a flexible and deformable self-pressurized elastic air pressure cabin scheme was designed based on the existing fixed volume pressure chamber, and its performance was evaluated. With Bernoulli's theorem and deformation coordination relation, a coupled model of internal pressure, nozzle velocity and thrust of water rocket was established. Moreover, the influence of different initial states (water volume ratio and inflation air pressure) on water rocket thrust was also studied with the numerical calculation method, and the thrust difference was compared and analyzed further between the fixed air pressure cabin and the elastic air pressure cabin under the same initial conditions. The research results show that the improved flexible and deformable self-pressurized elastic air pressure cabin could effectively increase the water jet velocity during the launch, and the thrust generated by the water rocket in the same initial state increases significantly by 46.95%. Important reference for improving the flight performance of water rockets and the optimal design of new flexible deformation water rocket scheme is provided.