Abstract: Validation of the experiment of gas-liquid two phase flow instability in vertical parallel channel was performed by the best estimate system computer code RELAP5. The results calculated by the non-equilibrium two-fluid model of RELAP5 agree well with the experimental data. Based on these results, the effects of main operating parameters on the system behavior were studied. The results show that the effect of subcooling of inlet is nonlinear, and system pressure can increase the stability of the system and reduce the amplitude of oscillator, and the increasing of the inlet throttle improves the stability. With the dissymmetry degree of dissymmetry throttling increasing, the total critical heating power of oscillation is increased. With the dissymmetry degree of heated power increasing, the total critical heating power of oscillation is reduced.