Abstract: When two liquid phases are mixed, it is important to know the variation of drops distribution for the research of mass transfer. Recently, the method mostly used is to set up a population balance model. But in the model, it is difficult to get the coalescence frequency not only from direct observation but also from mechanism modeling.Therefore, a spray-stirred mixer is set up to obtain instantaneous drops distribution. And the breakage frequency can be ignored when liquid drops is very small.There is only coalescence frequency that can affect drops distribution, which can be obtained from the instantaneous drops distribution. In this paper,the drops self-similarity is tested and drops coalescence frequency can be obtained by the inverse-problem method. Compared with the coalescence frequency deduced from theory, this method can give more exacted drops distribution.