The invention provides a rough mill processing method of a fir tree blade root, and solves the problem that the existing rough mill processing method has low processing efficiency and high cost. The rough mill processing method comprises the following steps of: (1) establishing a processing coordinate system of a four-axis horizontal type numerical control milling machine, and processing a driving auxiliary surface; (2) calculating a processing path along contour of the blade root fir tree and the contour of a midbody, and milling a blade root processing step surface by using a bull nose circular knife end mill; (3) calculating the processing path of a deep groove in milling the contour of the fir tree, and processing the deep groove of the fir tree blade root on the basis of the blade root processing step surface by adopting a circular knife disc milling cutter; and (4) calculating a layered processing track of a forming tooth-type knife, and processing the teeth patterns of the fir tree blade root by using the forming tooth-type knife.