Abstract: The calescence of water droplet in water-in-oil emulsion is studied in a rectangle coalescence pipe with laminar flow when it passes through an AC Field of 50Hz. Significantly studies the voltage applied on electrodes, the influence of the emulsion's velocity in electric field on the coalescence of water droplets and the water cut of the emulsion. The test result indicates that the water cut of emulsion has fewer influence on the coalescence and the others influence significantly.