Abstract: The flow instability in narrow annular multi-channel system was studied using RELAP5/MOD3.4 code. The sensitivity of the number of nodes and the number of channels was studied firstly with the aim to confirm the influence. The results show that the calculated results are sensitive to the number of control volumes, so before applying the RELAP5 code to the parallel channel systems, the number of nodes must be assessed. Based on optimal parameters, influences of inlet throttling, system pressure, inlet subcooling degree, length of inlet and outlet section, dissymmetry throttling on the critical power were obtained.