A needle valve of a diesel engine oil nozzle comprises an oil outlet end (11), a low-pressure end (13) and a guiding face (12). Cavities (20) are evenly distributed in the guiding face. The area occupancy of the cavities is 2%-30% of the circumferential theoretical superficial area of the guiding face (12). The concave depths of the cavities (20) in the radial direction of the needle valve are gradually increased from the position closest to the oil outlet end to the position closest to the low-pressure end. The cavities are machined in the guiding face of the needle valve, and the lubrication condition between the needle valve and a needle valve body is improved. Meanwhile, the sealing performance between the needle valve and the guiding face (12) of the needle valve is improved. The service life of the oil nozzle is prolonged. The invention further relates to a needle valve coupling piece with the needle valve and a needle valve machining method.