Abstract: Because bending and being tracked of sound line make torpedo tailing after target to smaller depth,upper limit control trajectory is usually considered in torpedo trajectory design.When torpedo arrives at the upper limit depth,upper limit control is performed compulsively,and then torpedo is pulled below the upper limit depth.The principle and trajectory feature of the upper limit control are analyzed,and the rule of upper limit control is presented.Moreover,the depth trajectories for torpedo to enter into the upper limit depth by three control modes during pursuing target are calculated and simulated.The results show that under the complicated upper limit control logic,the upper limit depth trajectory is not a simple form,but two or more irregular forms of re-entering into the upper limit depth,or regular oscillation form around the upper limit depth.These results may be expected to provide a theoretical basis for analyzing the problem in sea trial.