The invention discloses an internal thread milling method which is used for adopting a multi-axis such as a triaxial numerically-controlled machine tool to machine an internal thread and comprises thefollowing steps: mounting a rotary table on the triaxial numerically-controlled machine tool; mounting an angle head on the spindle of the triaxial numerically-controlled machine tool, according to the type of the internal thread, adjusting the axis of the angle head and the axis of the spindle to form a swinging angle, and locking the angle head; clamping a to-be-machined workpiece, and enablingthe axis of the bottom hole of the to-be-machined workpiece and the axis of the rotary table to be coincided; mounting a thread milling cutter on the angle head; controlling the rotary table to rotate at a constant speed and starting the spindle; and controlling the thread milling cutter to machine the internal thread in the bottom hole at the preset feeding speed and according to the preset cutter feeding path. With adoption of the internal thread milling method, the machining precision of the internal thread is improved, the surface smoothness of the thread is improved, the machining cost of the internal thread is lowered and the economic benefit is remarkable.