This paper investigates the effect of ageing on the mechanical properties and microstructure of building gypsum by setting different ageing condition. The results show that with the increase of aging time, the water requirement for the standard consistency of building gypsum decreases first and then increases, while the 2h strength increases first and then decreases. The higher the relative humidity, the shorter the optimal aging time. During the aging process of building gypsum, anhydrous gypsum absorbs water to generate hemihydrate gypsum, and the microcracks on the surface gradually self-heal due to the filling effect of dihydrate gypsum crystals generated by water absorption, and the porosity decreases, thus improving the mechanical properties; after the optimal aging stage, dihydrate gypsum continues to grow leading to a gradual decline in gypsum properties.