Abstract: The flow excursion accompanied with flow oscillation phenomenon under certain conditions is observed in the heating reactor test loop of the 5 MW reactor. Based on the experimental investigation, the entire process of flow excursion from its beginning, developing and converting into dynamic oscillation is described and the mechanism of sub-cooled boiling, condensation and flashing, which act on the process, is analyzed too. The analysis result shows: 1) the static flow excursion is a rather long process, and dynamic oscillation can be found in it; 2) in the course of the flow excursion, the flow instability is first dominated by the sub-cooled boiling and condensation, and then is dominated by the void flashing, at last is the type of oscillation controlled by itself; 3) the final oscillation is mainly the form of density wave oscillation, meanwhile has the characteristics of geysering stablility.