Many two-phase flow problems are quite complicated due to the existence of moving contact lines and shape transition of two-phase interfaces. Both numerical simulations and analytical study for these problems are very challenging, especially to quantitatively compare with physical experiments. In this talk, we will show that the Onsager principle could be used as a powerful approximation tool in these problems. This is illustrated by examples like sliding droplets on an inclined surface, capillary rising for a thin soap film, etc.