The invention belongs to the technical field of milling processing and discloses an algorithm for projecting cutter ring body to a side of a triangular plate in a five-axis machining path. The algorithm comprises the steps of (1) judging a special condition that the cutter ring body is projected to the side of the triangular plate, going to a step (5) if the special condition exists, otherwise going to a step (2), (2) determining an initial iteration angle according to a first geometric constraint condition, (3) calculating and dispersing an initial iteration angle area based on a second geometric constraint condition and a third geometric constraint condition so as to determine an optimal initial iteration angle, (4) searching an optimal angle by using a secant method based on the second geometric constraint condition and the third geometric constraint condition, and going to a step (5) if the search is successful, otherwise ending the algorithm, and (5) calculating a cutter contact according to the searched optimal angle. According to the algorithm, the three geometric constraint conditions are used to calculate an optimal initial iteration point and a precise search gradient, and the calculation efficiency of the cutter contact is improved.