Simplified model and performance calculation method were established to investigate performance of shockwave focus pulse detonation engine. Some main parameters were calculated to discuss their variation along with the inlet parameters and flight condition for an engine with a 90mm-diameter resonant cavity. The results indicate that thrust increases, and specific fuel consumption decreases with inlet total pressure increasing. With the increasing inlet total temperature, thrust and specific fuel consumption decrease. The increase in flight Mach number leads to increase in thrust and specific fuel consumption. The thrust and specific fuel consumption decrease with increasing flight altitude.