With the aid of transfer matrices method and personal computer a computational research has been carried out on the properties of a rectangular model representing the axial field of an electrostatic quadrupole triplet lens. The path of charged-particles beam traversing the rectangular fields has been determined by solving the trajectory equation of motion in Cartesian coordinates. A computer program has been written for this purpose. The optical properties of the electrostatic quadrupole triplet lens have been computed with the aid of the beam trajectory along the lens axis. Computer programs in Fortran 77 have been written for determining the various optical properties. The computations have been mainly concentrated on determining the focal lengths and spherical aberration coefficients in both horizontal and vertical planes of the trajectory along the optical axis. The results have shown that the choice of the geometrical and operational parameters depends on the function of the electrostatic quadrupole triplet lens in a particular electron or ion-optical system.