Abstract: By means of light scattering technology, Stability Index (SI) from Turbiscan Lab Dispersion-stability Analyzer can be used as quantitative indicator to evaluate the stability of preparing beta-cypermethrin 45EW, and the results from SI were in consistency with those of high temperature and freeze-thaw stability test. When SI of beta-cypermethrin 45EW was below 4.0, the emulsion system was found to be stable. Furthermore, the results of the orthogonal experiment indicated that the emulsifier ratio was the most important factor that affect the stability of EW, while co-emulsifier content and water quality had less effect, and the difference of the effects of co-emulsifier content and water quality was not significant. When emulsifier ratio was selected, water quality and co-emulsifier had synergetic effects on physical stability of beta-cypermethrin 45EW. Water with wide range of quality, including tap water, can be used to prepare the beta-cypermethrin 45EW.