In imaging electron optics, the electrostatic focusing concentric spherical system has a series of valuable properties, its potential distribution and electron trajectory can be represented by analytical form, and the electron optical imaging characteristics and its lateral aberrations can be quantitatively studied. Although there are many literatures on electron optical imaging, they are limited to the understanding of zero-order approximation imaging, and a lot of fallacies exist in it. This series of articles fully study the electron motion trajectory, electron optical imaging characteristics, and lateral aberrations in the electrostatic focusing concentric spherical system composed of two electrodes or multiple electrodes, investigate the image dispersion formed by the electron beam in the imaging segment, put forward some new idea and recognition, correct some fallacies in literatures, and establish my own theoretical system. The first article of this series mainly explores the electron moving trajectory under the electrostatic field in the concentric spherical electrostatic focusing system composed of two electrodes, deduces the new representations for the trajectory of electrons emitted from the photocathodes of the concentric spherical system with electrostatic focus of two electrodes in the polar coordinate system ρ=f(φ) and in the cylindrical coordinate system r=r(z), and explores the exact and approximate formulae of axial intersection position and its angle of inclination at the image section for the electrons emitted from the photocathode. This paper lays a foundation in studying the electron optical properties and lateral aberrations of the electrostatic focusing concentric spherical system.