NbMoTaW high entropy alloy powders were prepared by mechanical alloying. The effects of milling time on the powder phase structure, microstructure, impurity content and particle size were studied. X-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to analyze the phase and morphology of the high-entropy alloy powder. The element distribution and impurity content in the powder were quantitatively analyzed by the energy spectrum analyzer. The particle size distribution of the powder was measured by the laser particle size distribution tester. The results show that themixed powder undergoes three stages with milling time increasing: flattening, cold welding breakage, and sphericity.After 45 h ball milling, the powder formed a single BCC solid solution. The powder morphology changes from an initial irregular shape to a sheet shape, and then it changes into an ellipsoidal shape, and the sphericity is gradually optimized. The impurities mainly originate from the agate jar and the grinding ball. Different ball milling time periods have different impurity content growth rates. Particle size changes with milling time, showing a trend of increasing first and then decreasing, and the particle size distribution is more uniform.