This paper uses an improved charge density method （CDM） and a more realisticcathode operating mode to simulate the electron optical properties of field emission guns. In the paper a special 6-point quadrature formula has been derived to accurately calculate potentials near theemiter surface and a spherical diode model has been adopted with the cathode operating at the partial space charge limitation. The computational results show that the proposed method is applicableto the investigation of electron optical properties of field emission guns.