The invention relates to a demulsification, dehydration and separation method for emulsion. A hydrocyclone comprises a rotational flow chamber and an underflow pipe, the closed end of the rotational flow chamber is connected with an inlet pipe and an overflow pipe, the inlet pipe is tangent to and communicates with the circumferential inner wall of the rotational flow chamber, the overflow pipe penetrates through the closed end of the rotational flow chamber in an insulated mode to communicate with the interior of the rotational flow chamber, the other end of the rotational flow chamber communicates with the underflow pipe through a smooth transition pipe section, and both the rotational flow chamber and the overflow pipe are conductive. The method comprises the following steps that firstly, the emulsion is pretreated; then the overflow pipe faces upwards and is electrically connected with the positive electrode of a high-voltage pulse power supply, the rotational flow chamber is grounded; and then the pretreated emulsion is continuously input into the rotational flow chamber from the inlet pipe. According to the hydrocyclone used in the demulsification and dehydration method, the smooth transition pipe section is adopted for transition between the rotational flow chamber and the underflow pipe, so that the turbulence intensity in the hydrocyclone is reduced, a flow field is promoted to be stable, under the action of an electric field, coalescence of water drops in the emulsion is facilitated, the particle size is increased, and therefore the separation effect and the separation efficiency are improved.