The invention provides a tool for precisely machining an internal thread with variable groove width. The tool for precisely machining the internal thread with variable groove width comprises a cuttingblade, a movable sliding block connected with the cutting blade through bolts and a base connected with the movable sliding block through springs. The cutting blade comprises a blade body portion, amain cutting edge located on the upper edge of the blade body portion and side cutting edges located on two side edges of the blade body portion. The main cutting edge, the side cutting edges and theblade body portion are integrally formed. The circular arc radius of the main cutting edge and the side cutting edges is 0.1 mm. The circular arc transition radius of the side cutting edges and the blade body portion is 0.3 mm. The invention further provides a machining method for conducting precise machining on the internal thread with variable groove width by utilizing the tool. The tool and themachining method for precisely machining the internal thread with variable groove width have the beneficial effects that it is avoided that the cutting depth need to be adjusted constantly through adjusting fixture end displacement of machine tools in the process of turning the internal thread, and meanwhile, rough machining and precise machining of the internal thread are finished simultaneously.