The invention provides a machining method of a closed ternary grooving welding impeller. The machining method comprises the following steps that a wheel cover is machined according to the procedures of rough turning, semi-finish turning and finish turning; the outer diameter of an opening ring of the wheel cover is clamped, the wheel cover is aligned and leveled, and a blade on the wheel cover ismilled; after the blade is milled, secondary clamping for the milled wheel cover aligns the plane of the mille wheel cover; a wheel disc is machined according to the procedures of rough turning, semi-finish turning and finish turning; the wheel cover and the wheel disc are installed together, and the wheel cover and the wheel disc are welded and fixed in the circumferential direction; the initialangle of the milling groove of the wheel disc is determined; and a groove is milled in the wheel disc from the initial angle. According to the machining method of the closed ternary grooving welding impeller, a groove-shaped line of the wheel disc and a blade-shaped line milled by the wheel cover meet the precision requirement of the welding size.