Abstract: Density wave two-phase flow instability plays an important role in operation safety and reliability of heat exchange equipments. The mechanism of density wave oscillation was complicated. RELAP5 was adopted to simulate the flow instability and analyze the density, flow rate and pressure drop characteristics when the density wave oscillation occurred, and the two types of classical mechanisms were compared. The results show that flow rate, density and pressure drop oscillate periodically; the flow rates are different along the heated flow channel and the inlet and outlet flow rates oscillate almost out of phase. The single phase section pressure drop oscillates almost out of phase with two-phase section pressure drop; the density wave oscillation is mostly governed by time delay of flow perturbation propagation along the two-phase section of heated channel.