Phosphorylation of pectin was researched with solvent, reaction time, reaction temperature and ratio of phosphorus oxychloride (POCl3) to pectin as factors, and phosphorus content as index. T lymphocyte immune proliferation was tested to investigate the immune activity of pectin with and without phosphorylation. The experiment results indicated that dipolar solvent with higher polarity and reaction temperature could improve the phosphorus content; while it increased firstly and then decreased with the increase of solvent volume, reaction time, and quantity of POCl3. Under the optimal synthetic technological conditions, i.e. reaction temperature 50℃, the ratio of POCl3 to pectin 3∶1, and the reaction time 3h, the phosphorus content of the phosphorylated pectin could reach 11.52%. The apple pectin, phosphorylated apple pectins with high and low phosphorus content had no immune activity, while phosphorylated apple pectin with middle phosphorus content had obvious immunosuppressive activity.