Abstract: The experimental study was performed to investigate the flow instability in a natural circulation system by using deionized water, which gained the changeable situation of the flow rate in the narrow rectangle channel at the size of 2 mm and 5 mm. The analysis results show that the periods of wave with 2 mm and 5 mm narrow rectangle channel are 4.5 s and 5.7 s respectively, the symmetrical characteristic of the amplitude of 2 mm is not good as the 5 mm it is, and the periods of the 2 mm and 5 mm increase with the heating power. With the heating power increasing, flow excursion phenomenon happens in the channel at the size of 2 mm, and the characteristic of density wave weakens.