Spark plasma sintering (SPS) is an advanced sintering technology that has been recently developed in the world. Contrast to many reports about experimental investigations on the methods of new materials preparation, there are very few systematic studies on the special sintering mechanism of SPS technology. In the present paper, by using the pure electrolytic copper powders as the raw material, a series of sintering experiments have been designed and carried out. The evolution of the powder microstructures during SPS has been systematically studied, and for the first time a "self-adjusting mechanism" of the microstructure evolution is proposed, from which the essential for the advantages of materials preparation by SPS in the respects of high density, homogeneity and fine grain structure can be well understood. In addition, the changes of the relative density during SPS are quantitatively predicted by a theoretical model and confirmed by the experimental measurements.