Abstract: Surface of phosphogypsm was modified with stearic acid to solve the problem of phosphogypsum′s hydrophilicity and to improve its compatibility with the polymers.With activation index as target,optimum modification conditions were confirmed as follows:dosage of stearic acid was 2%of the mass of phosphogypsum;modification time was 20 min;modi-fication  temperature was 60 ℃,and pH was 7~8.Activation index of phosphogypsm reached 100% after modification and it became completely hydrophobic under optimum modification conditions.Unmodified and modified phosphogypsum were both tested with the experiments of extraction and infrared absorption spectrum (IR).Test results showed that the chemical bonds were formed between phosphogypsum and stearic acid.Preliminary industrial experiment results showed that modified phosphogypsum could be used as inorganic filler in PP resin,and its compatibility with PP was better.And mechanical  pro-perties of phosphogypsum/PP composite material has increased than that of pure PP. 