Abstract: A mathematical model was proposed for the density wave oscillation in parallel twin rectangular channels based on one-dimensional drift flux model in this paper. In the model, an empirical correlation recommended by Zuber was applied to calculate the void fraction of two-phase flow. Chisholm correlation and the correlation developed by NPIC were used for the prediction of the frictional pressure drop of two-phase flow. During the process of solving, mass, momentum and energy balance equations were decoupled and solved along the flow direction successively in the whole computational domain. The steady state calculation was made at first, then the periodical oscillation of flow was induced by adding small flow rate or heating power disturbance to the steady state solution. The flow instability boundary can be obtained by checking the oscillation mode in transient calculation. The calculation results indicate that the flow oscillations obtained through simulation have the same characteristics as those observed in the experiment. At last, the simulation were made for sixteen typical cases chosen from the experiment, and the results indicate that the relative differences between the calculated flow instability boundary heat flux and the corresponding experimental data are no more than ±20% for most part of cases.