Abstract: Technical details of using acoustic emission(AE) to monitor the critical structures during aircraft fatigue test were introduced. Due to the requirement for real time monitoring, parameter trend analysis(PTA) technique, assisted by amplitude-based and space-based filtering techniques, was adopted to analyze the signal in real time. Especially in the space-based filtering analysis technique, the proportion of the events from damaged area to the total events was utilized to analyze the state of crack, which was further verified by correlative PTA. Satisfactory result was obtained and success was made in predicting a crack germination which came from the third wall of right airfoil. These results provided important guarantee for the fulfillment of the fatigue test.