Abstract: The study on the two-phase flow instability of forced circulation parallel multi-channel systems was presented using the RELAP5 code. The results show that, at the low inlet sub-cooling number conditions, there are two different instabilities in parallel channel systems, the in-phase density wave instability and the two-phase flow instability between multi-channels. The effects of main operating parameters on the density wave instability were studied. And the stability boundary of the density wave instability was confirmed. Finally, the unstable regions of the density wave instability were determined.