Abstract: Two-phase flow instability has been researched in the past decades. The forced circulation vertical parallel narrow channel system under different flow conditions was performed by the best estimate system computer code RELAP5/MOD3.4. The instability of parallel channel system was studied. The results show that two-phase flow instability between multi-channels (FIBM) and flow excursion instability are two typical types of instability in parallel narrow channels. The process and inherent reason of flow excursion and FIBM in forced circulation parallel channel system were analyzed. The effects of main operating parameters on flow excursion instability were studied.