In this paper, the index of aggregate distribution uniformity, uniformity, was defined and the corresponding calculation formula was provided. Thereafter, a pixel-based digital algorithm for uniformity was established by using the discrete element principle and EDEM software, and the rationality and correctness of uniformity were verified by multiple numerical examples. The results showed that the uniformity proposed in this paper had a clear physical meaning, values of which were between 0 and 1, and the uniformity of aggregate distribution increased with the uniformity. The regularity of the calculation results by means of uniformity correlated well with those by other methods. Meanwhile, the uniformity was more in line with the people's conventional way of thinking, and could reflect the uniformity degree of particle distribution quantitatively and accurately in the mixing process. In addition, the mechanical interaction between aggregates in the mixing process was also considered. Therefore, some phenomena, for example, the decrease of uniformity caused by the floating of lightweight aggregate could be reflected by the uniformity index. Lastly, some problems in the formation of fresh concrete were analyzed and demonstrated, and the corresponding suggestions for practical engineering were also put forward.