Abstract: Flow instability may occur during the reflooding process driven by gravity and may hinder the continuous injection of cooling water because of the small flow rate of cooling water and the accumulation of steam in the pressure vessel, which has an important impact on the safe operation of nuclear reactors. A reflooding experiment device was set up and the variation regularity of flow instability under different resistances of steam outlet, high storage tank water level and initial temperature of the heated rods was studied. The results show that the flow instability includes cooling water initial injection, cooling water expulsion and cooling water re-injection. The steam’s emission rate and accumulation at the top of the experiment body depend on the resistance of steam outlet. Decreasing the resistance of steam outlet accelerates the emission rate of steam and prevents the accumulation of steam at the top of the experiment body. The peak value of pressure shows a drop and oscillation period becomes longer with the decrease of the resistance of steam outlet, which are beneficial to the stability of the system. Raising the water level accelerates the injection flow rate of the cooling water, increases the submerged ratio of the heated rods and reduces the frequency and duration of the flow instability. As the initial temperature of the heated rods increases, the cooling water flow changes from fluctuation to stagnation. The frequency and duration of the flow instability increase.