Tri (1-oxo-2,6,7-trioxa-1-phosphabicyclo[2,2,2] methylene-4) phosphate (TPMP) and microencapsulated TPMP (MC-TPMP) with a melamine resin shell, respectively, were synthesized. Their structures were characterized by IR, XPS, scanning electron microscopy (SEM), and thermogravimetry (TG) analysis. The flame retardance of TPMP and MC-TPMP combined with atactic polypropylene (APP) in polypropylene (PP) was characterized by limiting oxygen index (LOI) and UL-94 test. The authors found that the LOI values of PP/APP/MC-TPMP composites are higher than those of PP/APP/TPMP composites. They found that 30% of APP/MC-TPMP in PP gets 32.0% of LOI and UL 94V-0. The morphology of the char structure was studied by SEM. Thermal properties of the PP/APP/MC-TPMP and PP/APP/TPMP composites were investigated with TG and differential TG (DTG). The flame retardant mechanism has been discussed according to the results of TG, DTG, and SEM.