Abstract: The acoustic emission(AE) signal feature and its developing rule for active defect in crane beam was discussed. Based on a testing rig of simulating beam with three artificial cracks, the AE signals were obtained. The signals were collected under different loadings and various moments of crack from beginning to extending. Because of the complexity of the signals, the method of empirical mode decomposition(EMD) was introduced to decompose them. The results demonstrated that the AE signals of active defect were burst type and contained many frequency components in a burst. Generally the frequency was lower in the beginning of crack, with the developing of crack, the frequency became higher. The total frequency range of crack development was 25~160 kHz. The research laid the theoretical fundament for AE examination in active defect in crane equipment.