Abstract: Methodology of coastdown analysis for reactor coolant pump（RCP） was investigated with consideration of combined effect of loop fluid inertia and pump inertia. It has been verified that the calculated coastdown flow fits with the measured data better when considering the influence of loop fluid inertia. A coastdown performance oriented design method considering the influence of loop fluid inertia was developed based on coastdown analysis methodology investigation, which can increase the pump inertia flexibility without violation of nuclear safety. And the methodology of coastdown analysis and design can applied to sealless RCP when using overall efficiency without electric loss as input.