The combination of using acoustic countermeasures equipment is the hotspot in the field of torpedo defense of submarine. Based on the study of the combined use of decoy and the relative moving line method, the method of calculating the small caliber decoy’s initial course is given; Then, based on genetic algorithm, the technology of submarine torpedo defense by combination of decoys is proposed, which solves the problem that the amount of calculation of traditional exhaustive statistics method increases dramatically when the decision parameters increase, and realizes the ability that submarine can combine use the small and big caliber decoy to defend the incoming torpedo. The simulation results show that the technology can improve the submarine’s survival probability by more than 14%.