In this paper a simple method--surface-charge density method combined with multipole-field-expansion method--were developed to simulate the 3D electrostatic fields in the main lens with deviated aperture in color picture tube. Distributions of rotationally symmetric field, dipole field, quadrupole and sextupole field in different zone in main focusing lens were offered and the aberrations in our model were also discussed. This method appears to be promising for analyzing the relationship between the multipole fields and aberrations for more complicated electron optics systems.