With the heater pipeline leakage experiment conducted on the experiment table, inner pipeline leakage of heater was studied through the leakage failure detection system. The frequency distribution and the amplitude of acoustic emission signals of leakage versus internal pressure were analyzed. Variations of signals versus leakage aperture and dissemination distance were summarized. According to the mapping relationship between the leaking spot and acoustic emission signals, and between the leakage flow and acoustic emission signals, reasons of leakage failure were concluded. The results would be applied to heater leakage failure detection. And an on-the-spot detection in the power plant was conducted to test the heater under pressure in order to verify the results.