Underwater torpedo interception could be conducted by shell perforation or even warhead impact detonation with high speed projectiles. This work deals with the projectile water entry and sequent penetration into underwater torpedo whereby both aluminium shell perforation and explosive detonation are numerically studied. The water entry model and aluminum impact model are validated against test data for underwater aluminum shell penetration. Targets are located under water with 20 cm depth where aluminium shells for penetration are air-back while the impact detonation model has explosives inside shell. Four kinds of different nose shape tungsten projectiles are used for penetration simulation, revealing that ogival nose projectiles have superior performance for both shell perforation as well as detonation than conical nose projectiles. Numerical predictions shed some light on nose shape optimization design of high velocity projectile against torpedoes.