Abstract: The effect of temperature, ultrasonic and environment atmosphere on the stability of poly(pphenyl benzobisoxazole)(PBO) in strong protonic acid solutions, such as poly(phosphoric acid) (PPA) and methanesulfonic acid (MSA), was studied. Results show that in dark environment, PBO-MSA solution is sensitive to the temperature, whereas PBOPPA solution is stable even at high temperature. The degradation process of PBO on the MSA solution is consisted of two steps: first, the ohydroxy amide structure in the PBO molecule degrades; then, the benzoxazole ring openes and formes ohydroxy amide linkage structure, o-hydroxy amide linkage degrades subsequently. The degradation of PBO-MSA solution under ultrasonic belongs to stressinduced, which is consistent with the classic ultrasonic degradation mechanism. The results also show that PBO solution is stable under nitrogen, whereas the speed of photodegradation is fast when environment atmosphere is oxygen.