Toughness, strength, and rigidity of the composites can be improved simultaneously by filling plerosphere particles into polypropylene (PP). The mechanism of improving the mechanical properties of the composites is studied on the basis of the energy dissipation theory. By using scanning electron microscopy (SEM), the crack initiation, propagation, and termination on the plerosphere/PP composites are investigated by in situ tensile test. Observation results show that the particles can effectively lead to a number of microcracks instead of the breaking crack. These microcracks formation consumes tremendous energy. As a result, the toughness and strength of the polymer can be improved. Results of the in situ SEM observation and analysis to the dynamic process supply effective test evidence for the toughening mechanism of the plerosphere/PP composite on the basis of the energy dissipation theory.