Abstract: Open natural circulation systems are widely applied in the field of energy and chemical industries. Under some special conditions, open natural circulation systems with low height difference have to be applied, but little research focused on this type of systems. In this paper, the flow patterns of such a natural circulation system were experimentally studied at low power level by adopting the steam heating. In addition, the flow patterns and physical phenomena of the natural circulation system were analyzed in detail under different heating power conditions. It is found that there are four flow patterns under different power levels for the natural circulation system, because of the influence of non-condensable gas and subcooled boiling. The inlet and outlet temperatures of the heating section are analyzed in different flow patterns. The result shows that the outlet temperature of the heating section can be used as a judgment criterion of different flow patterns.