The invention provides a machining method for a T-shaped slot of a main shaft. The machining method comprises the following steps: acquiring a slotting position of the main shaft of an axial flow compressor; and determining a size region of the slotting according to the obtained slotting position of the main shaft of the axial flow compressor, a stored slotting position of the main shaft of the axial flow compressor and a corresponding relation between the obtained slotting position and the stored slotting position. The machining method has pertinence for selection of a slot of the main shaftof the axial flow compressor, showing that after the novel technological method performs rough machining on the T-shaped slot of the main shaft, a movable vane mounting slot is milled; a T-shaped slotfine turning tool is fed in the milled movable vane mounting slot, so the problem of low turning intensity of the T-shaped slot in the size is solved; and furthermore, easy tool relieving and damageto the tool are avoided, and the machining precision of a workpiece and the working efficiency are greatly improved.