Based on the preparation of excess-sulfate phosphogypsum slag cement （PPSC） with a large amount of wet grinding modified phosphogypsum paste and granulated blast furnace slag powder as the main raw materials， the excess-sulfate phosphogypsum slag cementitious road base material （PGBM） was designed and prepared based on mechanical properties， and its microstructure and mechanical properties were studied. The results show that the PPSC contains three main mineral components： mineral phase quartz （SiO₂）， dihydrate gypsum （CaSO₄·2H₂O） and hydrate product ettringite， and the mineral phase and microstructure affect the unconfined compressive strength and then affect the water stability of PGBM， but the growth rate of the unconfined compressive strength of the PGBM is consistent with the excess-sulfate phosphogypsum slag cement， and the erosion resistance of PGBM is also consistent with the development of its unconfined compressive strength. The base specimens of different proportions have good water stability， and their softening coefficients are all greater than 90%； when the content of PPSC is 5%， the 7 d unconfined compressive strength of PGBM can meet the specification requirements of pavement base strength； while the 15% PPSC content of PGBM shows a decrease in strength and increase in scour capacity at 28 d curing age. The reason is that the formation of excessive ettringite inside the PPSC causes the specimen to expand and crack.