A pot experiment was set up on using southern red soils of varying pH levels to cultivate upland rice with potassium humate as a passivator at fiveapplication rates(0.0%, 0.2%, 0.4%, 0.6%, 0.8%). Soil solution samplers were collected in situ using a soil solution sampler combined with sequential extraction methods, to study cadmium(Cd) speciation changes in both soil and soil solution. This approach was used to explore the stabilization effects and mechanisms of potassium humate on Cd-contaminated red soils with varying acidities. The results showed that after the application of potassium humate, the pH, soil organic matter mass fraction, and dissolved organic carbon mass concentration in the soil solution increased to varying degrees in both acidified and non-acidified soils, with the increase being proportional to the application rate. Compared with the control, after adding 0.8% potassium humate the exchangeable Cd molalities in acidified and non-acidified soils decreased by 30% and 20%, respectively; the carbonate-bound Cd molalities decreased by 21% in acidified soil but increased by 3% in non-acidified soil; the Fe-Mn oxide-bound Cd molalities increased by 14% and 39%, respectively; the organic-bound Cd molalities increased by 26% and 320%, respectively; the Cd2+ concentrations in the soil solution decreased by 89% and 95%, respectively; the proportion of Cd-OM increased by 33- and 48-percentage points, respectively; while the Cd content in rice grains decreased by 39% and 35%, respectively. These findings indicated that the addition of potassium humate increased the pH and organic matter mass fractions in both soil and soil solution, promoting the transformation of exchangeable Cd and carbonate-bound Cd into more stable Fe-Mn oxide-bound and organic-bound fractions, and also facilitated the formation of organic-bound Cd in the soil solution and the conversion of soil solution Cd into stable forms such as organic-bound Cd, thereby reducing the concentration of toxic Cd in the soil solution and ultimately decreasing Cd accumulation in rice grains. Additionally, the strong acid-neutralizing capacity of potassium humate made better efficacy in low-pH soils.