An experimental study on the viscosity-temperature characteristics and phase inversion process in aging oil emulsion was carried out. By using the static crude oil electrostatic coalesce experimental device, the effect of phase inversion on electrostatic coalescence and dehydration was examined. The effects of electric field intensity and frequency on dehydration were also studied in the case of aging oil emulsion ware passing through the high frequency and voltage electric field by using a specific electrostatic coalesce experimental device. The results showed that the water content at the phase inversion point was about 40%, the viscosity of aging oil emulsion increased obviously during phase inversion process, which hindered the dehydration. The dehydration efficiency was up to 97.8% under the optimal conditions of electric field intensity 1.25 kV/cm and frequency 2.5 kHz when the water content was 30%, the dehydration efficiency was only 4.2% under the same electric field parameters except 50 Hz frequency. High frequency electric field was more effective than regular electric field.