Abstract: In order to study the flame retardant effects of epoxy resin coated ammonium polyphosphate (EP@APP) microcapsules on polypropylene (PP), firstly, EP was used as the shell to coat APP by in situ polymerization method, then the EP@APP microcapsules were prepared, and it was compound with PP to prepare the EP@APP/PP composites. Secondly, solubility of EP@APP microcapsules was tested to explore the effects of process parameters on solubility. The water resistance was also investigated. With infrared spectroscopy, surface functional groups of EP@APP microcapsules were analyzed. Finally, limited oxygen index, tensile strength and thermogravimetric curves were tested and the thermal degradation kinetics of PP composites was analyzed. The results show that when the addition of EP is 10wt% of APP, and curing agent addition is 15wt% of EP, the completely coated EP@APP microcapsules were prepared by step heating method of maintaining 40 ℃ for 1 h, followed by maintaining 70 ℃ for 1 h. The solubility of the microcapsules in water is lower and the water resistance is good. After adding EP@APP microcapsules into PP, the limited oxygen index of PP composites is 35.5% and reaches V-0 burning class, the mass of residual char after burning increases, and the carbonize effect is better than that of PP composites which adding APP directly. Comparing with APP, the destructiveness of EP@APP microcapsules toward tensile strength of PP reduces obviously. The addition of EP@APP microcapsules makes the apparent activation energy of PP composites increases from 100.8 kJ/mol to 127.5 kJ/mol, and changes the thermal degradation oxidation process of PP composites, and a stable protecting carbon layer is formed by the generated residual char. The research results show that EP@APP microcapsules can improve the flame retardancy of PP composites effectively.