With the development and improvement of polymer processing technology, safety production requires higher and higher levels of reactor fault diagnosis and assessment. In this paper, a new method is put forward for the polymer reactor on-line fault diagnosis basing on acoustic emission (AE). During the loading process of the reactor, sensors detect the AE signals. According to the acoustic emission and location theory, the fault or flaws of the reactor can be found and located. Further, the assessment of the reactor can be got and it can guide the safety production of the reactor.