Trunnions are one of the core components of a steel ladle, active fatigue cracks may emerge in the trunnion roots for a long time hauling operation under high temperature condition. To overcome the difficulty of conventional ultrasonic method in detecting the weld of the trunnion roots, a inspection scheme based on acoustic emission (AE) plane location for the non-destructive testing(NDT) of the weld active defect of the trunnion roots was proposed. The original AE data was filtered and wavelet packet transform was used for the AE signal analysis. Results showed that the position of the weld active defect is located relatively accurate with the method. 3-layers decomposition and reconstruction was performed with the AE signal using wavelet packet, the frequency domain of active defect AE signal is mainly distributed from 62.5 to 125 kHz and the signal energy account for 81.25 % of the total energy, agreeing with the general frequency characteristics of the crack propagation.