In cementing operations, oil well cement needs to be mixed with admixtures to improve its performance. The early wet mixing method has shortages of long mixing duration, serious waste of raw materials and heavy environmental pollution, thus pneumatic dry mixing technology has drawn more attention recently. In order to investigate the variation law of mixing uniformity of oil well cement and admixture in dry mixing and determine the optimal operating condition. In this paper, the mixing process of oil well cement particles and admixture particles in the mixing tank is numerically simulated by using FLUENT and Euler-Euler multiphase flow mode. The effects of inlet air velocity, air inlet number, materials loading sequence as well as particle properties on mixing were analyzed. The results show that when the air velocity is too low, oil well cement and admixture unevenly mix. On the contrary, when the air velocity is too large, there will be a concentrated distribution of the particles. For the given case, the optimal air velocity is 1m/s. Moreover, increasing the number of air inlets can not significantly improve the uniformity of particle mixing. When loading the materials, particles with the highest density should not be placed at the bottom layer. The mixing performance is significantly affected by particle density, while particle size has no marked influence on mixing efficacy.