Change rule of active calcium content in persulfate phosphogypsum slag cement slurry was investigated. Through the comparison of test methods such as hydrochloric acid titration and EDTA titration, sensitivity analysis of test conditions, and research on reproducibility and repeatability of the test methods, a determination method for the active calcium content in persulfate phosphogypsum slag cement slurry was established. The results show that compared with the pH value, the content of active calcium can better represent the activity of persulfate phosphogypsum slag cement slurry and has more correlation with macroscopic properties of the hydration products. Since persulfate phosphogypsum slag cement slurry belongs to the poor calcium system with low content of active calcium, the determination method of hydrochloric acid titration is more suitable than EDTA titration. The repeatability and uniformity of hydrochloric acid titration are good, but it is sensitive to the mixing time and mixing temperature. It will be convenient that the mixing time should be 2h and mixing temperature should be 20℃.