Aquatic Unmanned Aerial Vehicles (AquaUAVs) hold extensive application prospects and significant potential value. Inspired by flying squids, water jet propulsion effectively addresses the challenge of underwater launches for AquaUAVs, enabling rapid and efficient takeoff. A novel high-power waterjet thruster using a gas generator as the high-pressure source is proposed. A mathematical model of thrust characteristics was established for this thruster to analyze parameter variations within the thruster during propulsion. CFD simulations of the waterjet process were conducted, and thrust characteristic experiments were performed. Discrepancies among the mathematical model predictions, CFD simulation results, and experimental data were analyzed to validate the correctness of the mathematical model. The thruster achieved a maximum thrust of 1,492 N and a total impulse of 219.1 N·s in testing, leading among water jet thrusters using the same principle. In the future, an improved prototype with multiple gas generators and optimized structure is expected to provide a solution for underwater launches of larger AquaUAVs.