To distinguish the sequence of events during power failure, it needs SOE records greatly. Through this, the operator can handle the incident, analyse and judge the power failure correctly. It needs to consider and resolve the problem that how to distinguish the dithering signals and ensure the SOE not misinformation. This paper analyses the methods of subtractive dithering signals, and points out characters of every method. A testing environment is set up, every method would be tested here. Finally, one subtractive dithering method for signals measurement is putforward to ensure the accuracy of SOE records.