The spray boundary and spray angle work as important characteristic parameters of spray field, which are mainly measured by image method. In image processing, the grayscale images are generally converted into binary images firstly. Then, the binary images are processed and calculated. Due to the multiphase flow characteristics of spray field, there is no judgement basis whether the obtained binary threshold and the image are consistent with the actual spray field. In order to digitally judge the spray field boundary, it was proposed to process directly according to the grayscale image of the spray. The mask templates were obtained and applied to the grayscale image, and then the gradient value of the grayscale image were calculated and compared by image morphology and iterative method. The spray field boundary and the spray angle were calculated by obtaining the grayscale image when the gradient value was maximum. Experimental results show that the proposed method provides a numerical judgement basis to judge and extract the spray field boundary through the gradient maximum value. Thus, the spray field boundary extraction and the spray angle fitting measurement are realized automatically by the program.