In order to investigate the gas jet flow field and thrust characteristics of solid rocket engine working underwater， an experimental study on the underwater work of rocket engine was carried out on the lifting platform connected to the hull. The expansion process of nozzle gas jet in open water was observed by high speed camera system， and the evolution process of underwater gas jet was obtained. The combustion chamber pressure and thrust of underwater rocket engine were measured. The thrust characteristics of the rocket engine under the water depth of 10 m， 30 m and 50 m were compared and analyzed. The experimental results show that when the engine is ignited underwater， the shape of gas jet is changed because of the interaction between water environment and gas， and gas liquid turbulent mixing is intense. With the increase of water depth， the combustor pressure is basically unchanged， the working thrust of the engine decreases. The relationship between thrust and water depth is nonlinear. Under the same water depth condition， the smaller the engine throat diameter， the smaller the thrust reduction， and the smaller the combustion chamber pressure， the smaller the thrust reduction.