Batch isothermal equilibrium experiments were conducted to investigate phosphate adsorption by red soil colloids with addition of citric acid, and effects of two different treatments, (A) removing or (B) retaining supernatant after the samples reached equilibrium on lead immobilization by red soil colloids. Results show that citric acid inhibited phosphate sorption on red soil colloids, and the effect intensified with increasing citric acid concentration. When the Langmuir equation was used to fit isothermal adsorption curve of phosphate on red soil colloids, the adsorption reaction constant (K), the maximum adsorption quantity (X_m) decreased with increasing initial concentration of citric acid. And the two treatments affected differently lead immobilization capacity of red soil colloids. On the whole, lead immobilization capacity was lower in Treatment A than in Treatment B, and increased with increasing initial P concentration and P adsorption rate, but, decreased with increasing citric acid concentration. In Treatment B, Pb immobilization capacity peaked when the initial citric acid concentration was 0.1 mmol L^-1 and initial phosphate concentration was lower than 0.4mmol L^-1, which indicate that in this condition, the coexistence of phosphate and citric acid jointly promote lead immobilization by red soil colloids.