Abstract: The harm degree of the rice blast and the rice sheath blight was detected according to the distance characteristic, at present there had not defined the only method of digital image distance.To gain harm degree information of crops disease in real time, this paper studied one method to examine harm degree of the rice blast and the rice sheath blight with ellipse model: used ellipse to fit the biggest sickness spot of the rice blast, calculated the length of principal axis of ellipse and the compared with manual examination the biggest sickness spot length, analysed both relevance, which had extremely the significance; Likewise, used ellipse to fit the length of paddy rice and the distance of disease apart from the base of the rice sheath blight, calculated two principal axis of ellipse length and asked the ratio, the average accuracy of measuring the harm degree of the rice sheath blight was upper to 90%. This study has greater reference value for the large-scale machine of plant protection to obtain the information of injury extent of plant diseases in real time.