In this study, 33 pieces of WSP were placed along three lines in a paddy. An M-18B Dromader AG aircraft flew and sprayed over the field, and the spray deposits were collected by water sensitive paper. Seven greyscale parameters were used to compare the color depth, deviation and homogeneity of digital water sensitive images. The greyscale images were converted to binary images with five threshold selection methods. The results of recognition of seven greyscale parameters and five threshold methods were compared to analyze the droplets in different scanned images on water sensitive paper. The effects of the threshold on the computation of deposit density and the stain size were evaluated. The most suitable grey scale was found to be luminosity. Finally, a manual validation was performed, and the relationship between the threshold and the stain size of was analyzed.