In this study，taking red soil upland under long-term fertilization(1988～2014)as research materials，total phosphorus(TP)and available phosphorus(Bray-P)as response variables，soil pH，the particle size composition of aggregates(PSAi)，iron and aluminum oxides，cation exchange capacity(CEC)，fractal dimension(D)as explanatory variables，the correlation between index variables and the changes of TP and Bray-P content in red soil upland were discussed by means of redundancy analysis(RDA)method.The results showed that long-term application of phosphorus or combined application of pig manure significantly increased TP and Bray-P content in red soil upland，while the application of crop straw had no significant effect.RDA found that the main controlling factors affecting phosphorus availability in red soil upland were soil pH，CEC，D，iron and aluminum oxide content and the distribution ratio of aggregates with particle size of 0.25～0.05 mm(PSA2)，0.001～0.005 mm(PSA4)，and <0.005 mm(PSA5)，among which pH and CEC played the most important roles.It can be seen that improving the acidic environment and increasing the cation exchange capacity of soil by long-term fertilization is the key to improve the phosphorus availability in red soil upland.