Abstract: The flow excursion instability in vertical parallel narrow channels under different flow conditions was performed by the best estimate system computer code RELAP5. The process and inherent reason of flow excursion in forced circulation parallel channel system were analyzed in detail. The effects of main operating parameters on the system behavior were studied. The results show that decreasing the inlet subcooling, the heat flux or outlet resistance, and increasing the system pressure, the hydraulic diameter or inlet resistance can depress the slope of the pressure drop-mass flux curve and avoid the flow excursion phenomenon.