Base on the monitoring fields of the 973 Project of ‘The Study on The Important Processes and Controlling Countermeasures in Farmland Ecosystem of China’, 28 soil samples were collected from the paddy fields derived from red soil in Yujiang County, Jiangxi Province. The soil samples were incubated for 3 h at 25℃ ± 1℃ to investigate the relationship between soluble P and Olsen-P under different rates of phosphate application. The Olsen-P content when soluble P was increased rapidly was supposed as the Olsen-P change-point of the soil. The results indicated that there was a significant correlation between Olsen-P change-point of the soils and background Olsen-P of the tested soils. The value of change-point of the studied soils was 114.22 mg/kg ± 7.86 mg/kg and soluble P was 13.12 mg/L ± 2.49 mg/L, and the application rate of phosphate when Olsen-P change-point occurred depended on active Fe content of the soil.