The invention discloses an ultrasonic image preprocessing system and method for diagnosing breast lumps, and the method comprises the following steps: S1, extracting a gland area of a breast image, and S2, obtaining gland distribution features of the gland region; S3, determining the mammary gland density of a mammary gland image according to the proportion of the gland area in the foreground partof the mammary gland image; grading the breast density; S4, determining the image type of the mammary gland image in combination with the mammary gland density and the gland distribution characteristics; S5, determining a preset mammary gland image processing mode corresponding to the image type, and processing the mammary gland image by using the mammary gland image processing mode; S6, extracting a gland region of the mammary gland image; S7, determining the subareas of which the ratios are greater than a preset value as gland boundary areas, and calculating a segmentation threshold of eachgland boundary area; and S8, taking the average value of segmentation thresholds of gland boundary areas as a segmentation gland area threshold of the mammary gland image, and segmenting the gland boundary areas of the mammary gland image to obtain gland areas.