Phosphogypsum slag cement is a low-carbon cement. The dissolution rate of slag in this system is slow, resulting in low early-age strength and long setting time. Reducing the water-cement ratio and using dry pressing molding method can improve its early-age properties by dense packing of particles. However, the very low water-cement ratio of dry-hard cement slows down the hydration process in long term. Therefore, the use of the additional sodium aluminate to promote the production of hydration products in early age and the hydration of slag in later age was proposed in this study. The effect of sodium aluminate on the compressive strength, hydration process, phase assemblages and micromorphology of phosphogypsum slag dry-hard cement were studied. The results show that the hydration process is promoted, and more hydration products are produced in early age resulting in the increasing compressive strength. The hydration of slag in late age is improved and the strength of sample with sodium aluminate is higher than reference sample in all curing ages. The optimized content of additional sodium aluminate is 0. 5% (by mass of total powder).