Abstract: An experimental investigation on the working fluid flow characteristics and the flow instability in the evaporator of Separated Type Heat Pipe（STHP）was presented.Flow patterns were observed and analyzed by visualization experiment.It was found that the bubbly flow,slug flow,and wavy foam flow occurred orderly in the evaporator with a small inclination angle when the heat flux increased.Two types of flow instabilities occurred,i.e.,the flow pattern transformation instability and the density wave instability.The effects of pressure,mass velocity,inlet subcooling,heat flux,and exit throttle on the flow instability were determined in the modeling experiment.The thresholds of the flow pattern transformation instability and that of the density wave instability were obtained.Non-dimensional correlations for predicting the flow instability in STHP were given by means of one-dimension single-fluid model.The results may be used for the engineering design of largescale heat exchangers of STHP with a small inclination angle in evaporator.