The etperiment Was performed pn the test loop (HRTL-5). which sinmlates the geometry and system design of a 5MW nuclear beating rcator In a Wide range of inlel subcoolings different flow features. such as single phase stable flow, sub-Cooled boiling stable flow, subcooled boiling staticpow excur-sion, density wave oscillalion and stable two phase flow in the mlnlral circulalion Svstem have been described The pheno menon and mecchanisnt of static flow excursion. which has never been smdied well in this field is especialIv interpreted. The erperimental results show that static flow excursion exists under certain inlet sttbcooling condilion in which only sub-COOled boiling occurs in the heated section In the process of flow excursion the mass flow rate and the inlef temperanlre decreases. while the exit temperature increases smoothly. As tbe process of excursion contimles for about 1 hour, bubbles enter in the flow. channel resulling in shorl period dynamic flow oscillalion which can only be seen in the process of this static flow excursion and has also never be studied well. These static arid dynamic flo instabilities combine ror about 2 hour. Then a point iS reached. at which the stalic, flow excursion stops, but the dynamic flow oscillation continues. The Study of flow excursion ond its concerned dynamic flow oscillation is of great significance for the delelopment of muclear heating reactor under natural circubttion.