Abstract In this study, a new method for predicting the hydrocyclone efficiency with the light dispersed phase has been developed. The equivalent volume method is employed to simplify the effective region of the hydrocyclone into a cylindrical shape, the influence of structural complexity on the method is reduced. Based on the analysis of the dispersed phase droplets and the assumption of determining the droplets been separated, a residence time model of the droplets in the effective separating region is theoretically developed. This method can be expressed as a simple function including the cyclone dimension and operating parameters. By compared with the previous experimental data, it is showed that the calculated value is close to the experimental value when the separation rate is more than 10%. When inlet velocity is not too large, cause the dispersed phase droplet breakage is not obvious, the applicability of this method is better.