Compound hydrocyclone is an important separation equipment in oilfield waste water treatment. In order to grasp the equipment separation characteristics, its pressure field and production ability research is becoming more and more important. In the process of pressure or pressure drop deduced, the hydrocyclone’s vortex field is divided into two parts: semi-free vortex area and compulsive vortex area. In the free vortex area, the pressure and the pressure drop are all deduced by the tangential equation, the pressure gradient equation and the relationship equation of tangential velocity in the hydrocyclone body and the velocity of the rotary crib. In the compulsive vortex area, the pressure and the pressure drop are deduced by the velocity equation and the hydrocyclone’s separation equation. As to the respect of the production ability, it is fixed on the relationship of the inlet flow-rate, overflow pipe diameter, the main diameter of the compound hydrocyclone, pressure drop and the inlet liquid density. The research indicates that the pressure or the pressure drop are all connected with compound hydrocyclone’s diameter, rotary crib’s running velocity and diameter of the maximum tangential velocity track face. As the results of the research, the ascertained key operators, pressure and the pressure drop, the hydrocyclone’s production ability can provide designing consult for the hydrocyclone designers.