A color cathode ray tube having an improved electron gun, includes a quadrupole lens consisting of an independent static electrode having a static voltage applied thereto and a dynamic electrode which is disposed opposite to the independent electrode and having a dynamic voltage applied thereto. A dynamic strong quadrupole lens effect is obtained by applying a modulation voltage or parabola voltage to the dynamic electrode. When electron-beam-passing-apertures in electrodes of a main lens are enlarged, each axial distance between the axes of three electron-beam-passing-apertures of the static electrode opposite to the dynamic electrode in the strong quadrupole lens forming electrodes is offset and the sizes of the opposite side electron-beam-passing-apertures are increased by the offset axial distance. Thus, a change in the convergence and spherical abberations resulting when a modulated voltage is applied can be minimized, thereby improving resolution characteristics in the peripheral areas of a screen and convergence characteristics.