In order to study the variable displacement characteristics of electrical servo variable displacement piston pump, whose the swash plate was driven by a DC servo motor through a transmission mechanism, the variable displacement designs of constant pressure, constant flow rate and constant power were achieved in this pump by controlling the DC servo motor. Based on AMESim, the simulation analysis of the above-mentioned variable displacement control designs were carried out. It has shown that by using the corresponding pressure and flow sensor signals and only through the DC servo motor control, the different variable displacement aims could be achieved in this electrical servo variable displacement piston pump and the actual demands for this pump could be met.