In order to deeply understand the applications of acoustic emission technology in the insulator on-line monitoring, based on the full understanding of acoustic emission testing technology principle, using the acoustic emission technology makes a comparative study of ceramic insulators and composite insulators uncleanness discharge. The experimental results show that although the experimental voltage of the two kinds of insulators exists high and low difference, the acoustic signals of filthy discharge basically have the same characteristics under the different degrees and experimental voltage, further proofed that the acoustic emission technology in polluted insulator discharging is feasible. In addition, in the polluted experiment extracts the actual characteristic quantities of acoustic emission signal, using the fuzzy reasoning method to process the actual data, the results show that the fuzzy reasoning can correctly judgment the strength of the uncleanness discharge and its threat level to insulator.