Abstract: In-service inspection of large above-storage tanks with acoustic emission technique is accepted by petrochemical industries more and more. But the way of acoustic emission signal acquisition is nearly same for different type storage tanks while their structure differences are neglected. The way that the guard sensors are used to help acquire the corrosion acoustic emission signals from the floor of the fixed roof tank is presented. Tow group sensors are placed around the tank shell in acoustic emission inspection for the fixed roof tank. One group is used to collect the noise signals from fixed roof when liquid drops fall to the surface of tank content, i.e. guard sensor, the other group is used to collect the acoustic emission signal from the floor. A special method is adopted to filter the signals by liquid drops. In order to evaluate the performance of the method, an in-service field inspection is practiced on product oil fixed roof tank. The result of the experience shows that our method can filter the noise signals by liquid drops and provide reliable original data for acoustic emission signal processing.