Phase partition of acrylonitrile in the vinylidene chloride/water system at different temperature and under pressure was studied. A calculation method for average VDC-AN copolymer composition with AN phase partition considered was proposed. The calculated results are in good agreement with the experimental data nearly the entire conversion range. VDC-AN copolymer with narrower composition distribution can be prepared in the suspension process and interpreted with dynamic equilibrium of AN between the oil and water phases continuously.