Abstract: In order to study the Ledinegg flow instability in rectangular narrow channel and analyze the main factors that impact the flow instability of the system, a series of experimental studies and mathematical theoretical derivations were carried. The experimental results show that with the increase of heat flux or the decrease of system pressure, the slope of internal characteristic curve in the negative slope region decreases. The smaller slope results in a bigger possibility of system instability, the bigger fluctuant amplitude of flow excursion. Different from conventional channel, the impact of subcooling on Ledinegg flow instability in rectangular narrow channel is small. Considering only the frictional pressure drop, and ignoring the acceleration pressure drop and gravity pressure drop, a mathematical correlation of the internal characteristic curve in narrow channel was deduced and the mathematical conditions of a stable system were provided.