A selective laser melting method for manufacturing the composition gradient material part was investigated to solve the problem of manufacturing metallic gradient material parts with complex structures. A composition gradient design combined with several groups of scanning path data files and a material txt format file was proposed to obtain additive manufacturing data of the composition gradient material part. A real-time powder mixing and distributing device was used to mix and distribute the composition gradient powder by swinging a hopper along two axes. The principle of flexible cleaning and recycling of powder was used to clean and recycle different powders in the same layer during the manufacturing of the gradient material part through selective laser melting. The experimental verification was carried out using a self-developed selective laser melting system for forming of gradient material part. A 4340+CuSn10 gradient material part was manufactured during the experiment, which showed a significant gradient change in color. Energy-dispersive X-ray spectroscopy analysis on the front side and the upper surface of the part showed that the average mass percentages of Fe in the middle three gradient areas were 4.94%, 36.49%, and 59.16% in the vertical direction, and 12.88%, 41%, and 53.59% in the horizontal direction. The composition of the experimental sample gradually changed in different layers and different regions of the same layer, which verified the feasibility of the method and provided a new choice for the manufacturing of composition gradient material parts.