Abstract: Based on the calculation method of void fraction of Bankoff’ variable-density single-fluid model for two-phase flow inside circular channel and parallel wall-surfaces channel, and the utilization of hydrodynamics method, the equation of velocity field of two-phase flow inside rectangular channel was established, and the calculation formula of void fraction about variable-density singlefluid model for two-phase flow in rectangular channel was deduced. The examples of numerical calculation were given, and the calculation results were compared and analysed for three kinds of channels. It is seen that the calculation result is in agreement well with the former Bankoff model.