The airborne PMS-detecting data of stratiform clouds in Yan’an in Sep.,2003 are analyzed.It is found that one density distribution function can represent the water drops size distribution in the warm layer of stratiform clouds in Yan’an.The function is used to fit the size distributions of small cloud drops,big cloud drops and raindrops.The fitted spectrum distribution is well matched with the observed one,and the correlation coefficients of their characteristic values are greater than 0.945.The calculated mean diameter,square-root diameter,cubic root diameter and concentration are also close to the observational results and the fitted size distribution is able to reproduce the feature of observed size distribution well.