Abstract: In order to discuss the possibility of torpedo(diameter: 534 mm) swim-out launching by the submarine torpedo tube in service, an interior trajectory model was set up in consideration of different resistances and the fact that the area ratio of the circular clearance cross-section of torpedo tube in service to the torpedo cross-section is small. The pressure difference of seawater compensation was defined and added to the model as a resistance parameter, and the launching process was analyzed. Based on the abnormal limited boundary condition in the swim-out torpedo tube, simulation was conducted to calculate the resistances of pressure difference and the flow losses according to different movement phases of torpedo in the constant section tube. Comparison of the test and simulation data verified the validity of the mathematical trajectory model, and proved that the current submarine torpedo tube could not launch the 534 mm torpedo.