Phosphorus deficiency in soil limits crop yields, especially in calcareous soil in arid and semi-arid areas of the Loess Plateau. Brassica napus L. can activate insoluble phosphorus in soil; however, the specific inorganic phosphorus components activated by rapeseed remain unclear. Field experiments were conducted in 2020, with rapeseed planted during the summer leisure period, to investigate the effect of rapeseed on phosphorus availability both before and after returning to the soil. Rapeseed was returned to the soil at two timings prior to winter wheat sowing: at flowering (F0) and 10 days after flowering (F10). The following treatments were applied: CK (no plants or fertiliser), N0P0 (no fertiliser), N150P0 (only N fertiliser), N150P60 (N + P2O5 [60 kg/ha]) and N150P120 (N + P2O5 [120 kg/ha]). Planting green manure during the summer fallow period after winter wheat harvest. Although rapeseed cultivation depletes the soil inorganic phosphorus pool, its incorporation increases soil total phosphorus and available phosphorus content by 14.3%-38.9%, respectively. The N150P60F0 treatment significantly enhances soil available phosphorus and alkaline phosphatase activity, thereby promoting phosphorus availability. After incorporation, the content of more active Ca2-P and Ca8-P in the soil increased, whereas the less soluble phosphorus fractions such as Al-P, Fe-P and O-P decreased. Early incorporation helps reduce the formation of poorly available Ca10-P, thereby enhancing soil phosphorus availability. N150P120F0 significantly increased soil bacterial and actinomycete counts, whereas fungal abundance was highest under the N150P60F0 treatment. Bacterial abundance showed a significantly positive correlation with soil available phosphorus and active phosphorus fractions (Ca2-P, Ca8-P), suggesting that bacteria play a key role in the phosphorus activation process driven by rapeseed straw return. Our results provide a theoretical basis for improving soil properties and enhancing soil phosphorus availability through rapeseed returning, thereby promoting efficient phosphorus utilisation by subsequent crops.