Abstract: In the field of grouting engineering, ordinary cement slurry is most widely used, and the injectability of slurry is one of its core performance indexes. Improving the injectability of cement slurry can not only broaden its application range but also reduce the use of expensive grouting materials and lower the cost of grouting. In the previous indoor experiment, it was found that the injectability of cement slurry increases and then decreases with hydration, with a peak at 60 minutes of hydration time. In order to reveal the mechanism of injectability change, this study used a polarized light microscope to continuously observe the structural change of cement slurry during hydration, tested the change of internal particles by an acoustic laser particle sizing instrument, and tested the change of macroscopic viscosity. The mechanism of time-varying cement slurry injectability was analyzed in the framework of relevant grouting theory. The results showed that: (1)particle dissolution and crystal precipitation caused the internal structure of the cement slurry to change continuously, with particle reduction by dissolution and strengthening of the connection between particles by crystal precipitation; (2)the dissolution rate of cement particles was 0.005 μm ·min^-1 within 120 minutes of hydration, while the viscosity of the cement slurry increased exponentially; (3)dissolution of the cement particles, aggregation of the cement particles, and growth of the viscosity of the cement slurry. The three together determine the change of cement slurry injectability; the dissolution of cement particles in the early stage of cement hydration is the main cause of the increase of cement slurry injectability, and the aggregation of particles and the decrease of fluidity in the late stage of hydration are the main causes of the decrease of cement slurry injectability. This study is valuable for the fine utilization of ordinary cement slurry.