Abstract: The electrostatic coalescer is superior to traditional electric dehydrator in terms of both dewatering effect and energy consumption, but the strong fluctuation of its power system may have certain impacts on its operation. To determine the main factors for the power system signal fluctuation and their influences on the electrical signals, this paper presents an experimental study on startup characteristics of the power system of built-in electrostatic coalesce and electrical test equipment, based on the electrostatic coalescer theory and the basic law of RLC circuit. Moreover, the double-insulated electrode plates are used to change the voltage, flowrate and water content, so as to identify the fluctuation characteristics of electric signals. The results show that the flowrate and water content of the emulsion have higher impacts on electric signal fluctuation - the higher the water content and the greater the flowrate, more severely the current fluctuates at startup. By changing the startup method, the current fluctuation can be mitigated to improve the stability of the coalescer.