Abstract: The geysering mechanism was analyzed based on experiments. The analysis results show that the geysering is a result of interaction between the buoyancy and resistance, for which the non-uniform of heat generation and distribution in the system is an important precondition. The fluctuation of flow parameters in the riser is caused by the interface oscillation of the vapor accumulated in the heating section and the liquid in the riser. The intermittent vapor venting would occur after the interface ruptures. Based on the above mechanism, the system heat transfer models of geysering were developed. The results calculated with these models show good agreement with the experimental data.