PPTA fibers made by China ShenMa Group Co., Ltd and Kevlar49 fibers made by Dupont as experimental materials were used in this paper. The differences of molecule structures were contrasted through infrared spectrum and degree of polymerization-counting by the detections of viscosity. The PPTA fibers were put into distilled water and seawater made by ourselves respectively, then ultrasonication were used to deal with them. After this, using the SEM to observe, found that the cracks appear on the fibers surfaces, presented the fibrillation and plush-like microfine fibers. Meanwhile the strength weak-link increased and breaking strength and elongation at break of fibers lowered after ultrasonication. Molecular kinetic theory was used in this paper to study loss of fibers the mechanical property.