To achieve precision mosaic of sprinkling width in wide spraying mechanism, a novel auxiliary GPS navigation method based on machine vision was put forward. Firstly, the foams sprayed to the breadths of spraying mechanism were identified, during which blue foams were chosen in order to effectively segment the target and background, and then the center line of foams image was obtained by iterative least square method after extrablue gray algorithm, morphological filter, anchor points selection and Otsu segmentation. Secondly, the definition of rotate angle and offset distance was given, and the information of course deviation was obtained by analyzing the parameter of foam’s center line. Experiments showed that the proposed method could accurately obtain the information of foams. The mean error of rotate angle between the values obtained by the proposed method and the real values was 1.58°, and the maximum value was 2.5°. The mean error of offset distance between the values obtained by the proposed method and the real values was 5.4cm, and the maximum value was 8.4cm.