A new mixing rule for van der Waals-type two-parameter cubic equations of state (EOSs) has been proposed in this work, which is particularly suitable for highly asymmetric systems. The new mixing rule has been applied to the PRSV and SRK EOSs to calculate vapor-liquid equilibria (VLE) of polymer solutions. Compared to the other two mixing rules which have been used for VLE calculations of polymer solutions, the new mixing rule with only one temperature-independent parameter is very accurate for a wide range of temperatures. As the new mixing rule is very simple and accurate and only one temperature-independent parameter is needed, it enables the two-parameter cubic EOSs to be used in the practical calculations needed in the industrial processing of polymer solutions.