Due to the inconsistency of dimensionless pollutant transverse diffusion coefficients in a physical model and its corresponding prototype river, the similarity law for the distorted physical model on water pollutant diffusion was investigated. It was found that the transform relationship of dimensionless transverse diffusion coefficients between the distorted model and the prototype river was related with the distortion scale ratio, αyp=αym·λl/λh， i.e. proportional to the horizontal scale and inverse proportional to the river scale. Validated by the experimental data between a prototype and a model, the result can provide guidelines for lab experiment in the research of transverse mixing of pollutant.
