Abstract: According to the simulation loop of full power natural circulation integrated-type reactor, the mathematical model taking account into heaving movement was established. By means of programming to solve the conservation equations, the mechanism of heaving on natural circulation was analyzed. The results show that heaving plays an important role on the natural circulation. Larger amplitude or longer period will result in more wide-coverage fluctuation. Natural circulation flow under heaving conditions ensues from both the alternating force and the alteration of density distribution. Under some certain conditions, heaving is likely to lead more serious consequence than rolling, and need to pay more attention.