Based on Stokes Law,settling velocity and ratio of settling velocity of cation and anion exchange resin in mixed bed are calculated and analyzed.It shows that the settling velocity and the ratio of settling velocity differ greatly and this results in different resin separation effect if cation and anion exchange resin according with the standard is different in particle size and density.In order to determine its separation effect,the distribution of particle size settling velocity as well as ratio of settling velocity of cation and anion exchange resin should be considered comprehensively in choosing and evaluating the exchange resin in mixed bed