To explore the regulatory effects of phosphorus fertilizer (KH?PO?) 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 and granite. Ten treatments for each soil included: different amounts of chemical phosphorus fertilizers (0 (CK), 5 (P1), 10 (P2), 15 (P3), 25 (P4), 50 (P5), and 100 mg/kg (P6)), straw (20 g/kg, S), and their combined application (P5S, P6S). The results showed that as compared with CK, the pH of Quaternary red clay soil in P5 and P6 treatments increased by 0.10 to 0.17 units, and the pH of granite soil in the P6 treatment increased by 0.08 units in the end of the incubation; the single-straw application or combined application of straw and phosphorus fertilizer treatments increased the pH of Quaternary red clay soil and granite soil by 0.12 to 0.17 and 0.98 to 1.05 units, respectively. Compared with CK, under P1-P5 treatments the content of exchangeable aluminum in Quaternary red clay soil significantly increased by 7.3% to 13.9%, but there was no significant change in the P6 treatment; all phosphorus application treatments significantly reduced the content of exchangeable aluminum in granite 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 Quaternary red clay soil and granite 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 types of soil. Therefore, maize straw can effectively reduce the exchangeable aluminum content in red soil, and its effect is better in red soil derived from granite than that from Quaternary red clay; the effect of corn straw in reducing the exchangeable aluminum of red soil was much better than that from phosphorus fertilizer; soil pH elevation, promotion of aluminum phosphorus and middle stable organo-Al complex were the main reasons for the reduction in exchangeable aluminum content.