In order to obtain the gradient material composition of parts easily in the part forming process, a selective laser melting additive manufacturing system for gradient materials was developed by using the real-time powder mixing method which integrates multiple powder mixing actions such as impact mixing after synchronous discharge powders, converging mixing through a conical cavity, and air flow mixing. With this system, the experimental verification of gradient material forming was carried out, and the forming effect was analyzed combined with the sample photos, microscopic images, and energy spectrometer detection. The result shows that the gradient materials can be freely added in the horizontal and vertical directions to produce gradient material parts with complex shape and structure in the system. The micro areas composition analysis of the gradient material zones of the gradient material part shows that the dispersion degree of the average mass content of elements in each micro zone is small. The variability coefficient of the main elements in each micro zone is not more than 0.59 when the average powder mixing time of each layer is 10s~15s during selective laser melting process, achieving good mixing uniformity.This research provides a new way for free manufacturing of gradient material parts.