Different parameters of swim-out torpedo launcher have different effect on the interior trajectory of torpedo. Based on analyzing the strength condition of the torpedo in variable section swim-out launcher, the interior trajectory of torpedo was modeled. Then, the interior trajectories of torpedo were simulated under the condition of different structural parameters. The result shows that the inner diameter of back section mainly affects time of torpedo swimming out, and the inner diameter of front section affects it's time and speed much. So, the inner diameter of front section should be given much consideration in design.