Abstract: An instability theoretical model for multi-channel system had been developed by building one-dimensional homogeneous model and adopting nonlinear analysis for two-phase flow in vertical single pipe,supplied by Clausse and Lahey.With the same parameters as the experiment,two-phase flow density wave oscillation in parallel inclined inner ribbed pipes was analyzed and solved in this paper.The variation of the working fluid inlet velocity perturbation with time was found using the time domain method.The criteria of density wave oscillation were defined according to the convergence or divergence of the velocity.The pulse cycle of calculated results was mostly between 5 and 16 seconds,which was quite similar to the data between 7 and 19 seconds in the experiment.It is shown that the calculated results have a good agreement with the experimental data.Therefore,the method can be used to determine whether the incidence of density wave instabilities.It can also provide a reference for the safety parameters of the water wall in the supercritical pressure boiler.