Abstract: Large power reactor is a trend in reactor development because of better economy. AP1000 introduced by our country is a two-loop passive safety design. A concept scheme of three-loop passive safety reactor was given out in this study based on AP1000 reactor system design with necessary modifications of reactor core and safety systems. And the RELAP5 analysis model of this three-loop concept was established. Steady-state calculation and typical LOCA analysis were employed. Typical accident sequences and transient response curves are given out in this study. And the three-loop passive safety reactor concept is proved to be reasonable preliminarily.