To explore the regulatory effects of phosphorus fertilizer (KH₂PO₄) and straw on exchangeable aluminum in red soil derived from different parent materials, a constant-temperature incubation experiment was conducted to determine the changes in soil pH, exchangeable aluminum, organically bound aluminum, and phosphorus availability and their interrelationships. The soils were derived from Quaternary red clay (red soil) and from granite (latosolic red soil). For each soil, ten treatments were established: different amounts of chemical phosphorus fertilizers (0 (CK), 5 (P1), 10 (P2), 15 (P3), 25 (P4), 50 (P5), and 100 mg/kg (P6)), maize straw (20 g/kg, S), and their combined application (P5S, P6S). The results showed that, compared to CK, the content of exchangeable aluminum in red soil significantly increased by 7.3% to 13.9% under P1 to P5, but there was no significant change under P6. All phosphorus application treatments significantly reduced the content of exchangeable aluminum in latosolic red soil, and the reduction increased with the increase in phosphorus application rate, ranging from 23.1% to 54.1%. The single-straw application or combined application of straw and phosphorus fertilizer treatments decreased the content of exchangeable aluminum in red soil and latosolic red soil by 22.8% to 33.3% and 89.1% to 95.9%, respectively. Random forest model analysis indicated that pH, middle stable organo-Al complex, and aluminum phosphorus had the greatest impact on the content of exchangeable aluminum in the two soil types. In conclusion, maize straw can effectively reduce the exchangeable aluminum content in red soil, with a better effect than phosphorus fertilizer, and its effectiveness is more pronounced in latosolic red soil compared to red soil. The reduction in exchangeable aluminum content by adding phosphorus fertilizer and maize straw is primarily due to increased soil pH and the promoted formation of aluminum phosphorus and middle stable organo-Al complex.