The invention relates to a machining method for a helicopter fairing supporting piece. The method includes the steps that firstly, preprocessing is conducted; secondly, a positioning clamp is installed at the clamping installation location 1, areas except for a middle pressure plate 2 and the clamping installation location 1 are subjected to rough turning, and the allowance is 1mm; thirdly, the middle pressure plate 2 is removed, four lateral clamping plates 4 are installed, a part 3 is fixed from the outer side, and the residual unprocessed areas in the first step are subjected to rough turning, finish turning and finish milling; fourthly, the part and the positioning clamp are connected through cooperation of a sunken hole machined in the second step and a corresponding threaded hole in the positioning clamp, the lateral clamping plates 4 are removed, the residual area of secondary finish turning is subjected to finish turning, and the residual portions, except for a lightening hole, of the part 3 are machined; and fifthly, an outer end pressure plate 5 is installed at the outer end face of the part 3, and the lightening hole in the outer end pressure plate 5 is subjected to rough milling and finish milling. The clamping purpose is reasonably achieved in machining, resources of tools and numerically-controlled machine tools are saved, cycling time is greatly shortened, and a good effect is achieved.