Abstract: This paper studies the variation of acoustic emission(AE)characteristics during dacite failure and the rules of AE b value in rock failure precursor. We carry out tests of dacite samples under uniaxial loading, triaxial loading and triaxial unloading conditions. The AE is detected at the same time. The results show that:the change of AE b value is closely related to the stress path. There are significant differences between loading and unloading paths, but the uniaxial loading and the triaxial loading are similar. In uniaxial and triaxial loading tests, the confining pressure has great influence on the change of AE b value. Before rock failure, for the overall AE b value, the higher the confining pressure, the bigger the value. When rock is damaged, the higher the confining pressure, the greater the decline of b value, dropping more rapidly. The b value also reaches its minimum in the whole process. In triaxial unloading test, the effects of confining pressure on the b value is not obvious. Compared with the triaxial loading test, when triaxial unloading tests reach a certain unloading level, the overall b value is lower, and the minimum value of b value is smaller at last. We propose that when rocks reach peak strength and there is a significant stress drop subsequently. The rapid dropping of b value can be used as a precursor to damage. The threshold of b value is determined by a large number of indoor rock AE test results. Rapid drop of the AE b value and b value breaking through 0.6 can be used as the precursors of rock failure.