In order to meet the needs of miniaturization and large field-of-view (FOV) of endoscopes in modern medical field, by using the retrofocus optical structure as initial structure, a high definition electronic endoscope objective optical system with 5×5 microlens array in the visible spectrum designed by Zemax software is presented. The optical system has a FOV of 110°, a focal length of 1.55 mm, F number of 4.2, a maxium clear aperture of 3.15 mm and a total length of system of 7.99 mm. The optical system is composed of 6 pieces of lens, which are divided into the front lens group and the rear lens group. The front lens group includes one lens with two-array surfaces. The rear lens group includes one double cemented lens. The modulation transfer function value of the whole system is 0.36 at the spatial frequency of 120 lp/mm, which is approaching to the diffraction limit, and meets the use of medical treatment. The design results show that the microlens array has obvious advantages in increasing the field of view of the endoscopic optical system without changing the aperture, and has a wide application prospect in medical treatment.