Abstract: Weld fatigue crack constraints life of steel structure. The monitoring of crack initiation, development process and the impact factors has great significance. In this paper, the acoustic emission monitoring of weld cracking, crack activities was identified in the complex environment noise signal. By analysis of the causes of fatigue crack damage as well as materials in alternating loads under the mechanism of acoustic emission the signal characteristics for characterizing the weld cracks activity process were obtained. It is shown that acoustic emission technique is rather sensitive to the steel crack damage activity and strength, and acoustic emission can achieve real-time monitoring of the development and changes of crack damage state.