Abstract: The macroscopic flow behavior and rheological properties of cemented paste backfill (CPB) are highly impacted by the inherent structure of the paste matrix. In this study, the effects of shear-induced forces and proportioning parameters on the microstructure of fresh CPB were studied. The size evolution and distribution of floc/agglomerate/particles of paste were monitored by focused beam reflection measuring (FBRM) technique, and the influencing factors of aggregation and breakage kinetics of CPB were discussed. The results indicate that influenced by both internal and external factors, the paste kinetics evolution covers the dynamic phase and the stable phase. Increasing the mass content or the cement–tailings ratio can accelerate aggregation kinetics, which is advantageous for the rise of average floc size. Besides, the admixture and high shear can improve breaking kinetics, which is beneficial to reduce the average floc size. The chord length resembles a normal distribution somewhat, with a peak value of approximate 20 μm. The particle disaggregation constant (k₂) is positively correlated with the agitation rate, and k₂ is five orders of magnitude greater than the particle aggregation constant (k₁). The kinetics model depicts the evolution law of particles over time quantitatively and provides a theoretical foundation for the micromechanics of complicated rheological behavior of paste.