The invention discloses a machining method for a semicircular thin-walled part. The machining method comprises the steps that machining burrs of the semicircular thin-walled part are removed, the partis perpendicularly placed on an equal-height iron cushion of a rotary workbench, an arc bottom is located on the equal-height iron cushion, and the part is pressed with screws and pressing plates after an A plane is located on the horizontal plane; a cutter, a measuring tool and a wall thickness meter for machining are placed on working position apparatus; a numerical control program is programmed according to the part clamping position, the rotary center is set as a program coordinate origin according to the principle that the relative distance between the rotary center of the rotary workbench and the circle center of the part is invariable, and a part coordinate system is determined; and the outer edge of the part is vertically milled with the cutter, and the part is machined with the bottom edge of the cutter in an up-bottom cutter moving mode. According to the machining method for the semicircular thin-walled part, the product quality can be guaranteed, and the working efficiencyis significantly improved.