The plane microlens array is a plane array that is buried under the surface of the flat glass substrate,which consists of several microlens elements with uniform geometric dimensions and a three-dimensional gradient of refractive index according to a certain rule. There are many methods for making GRIN planar microlens arrays, including hot embossing technology, ion beam etching, the melted photoresist method, and so on. Each of these methods has certain advantages and limitations. The hot embossing technology has high pattern reproduction accuracy, but the demolding process is complicated. Using ion beam etching to fabricate microlens has good surface microstructure, but the production cost is high and production efficiency is low. The melted photoresist method has a simple manufacturing process, but the planar microlens array made by this method has poor thermal stability and is prone to aging. The fabrication process of GRIN planar microlens array using ion-exchanging method is relatively simple. The size of the lens element fabricated by ion-exchanging method can be less than 10 μm, the focal length can be less than 0.1 mm, and the thickness can be less than 0.1 mm. Moreover, the array structure and imaging uniformity are good. It has been widely used in artificial intelligence, portable equipment, integrated imaging, three-dimensional imaging, beam homogenization, beam shaping and other fields that require small-sized array optical elements. In this paper, circular aperture gradient index plane microlens arrays with different diameters are fabricated by ion-exchange technology and photolithography. By sampling in 6 time intervals during the ion-exchange process, the ion-exchanging depth and width of microlenses with different aperture sizes, the focal length, distortion and numerical aperture of lens elements were measured. The ratio of ion-exchange width to depth decreases with the increase of ion-exchange time, and the average diffusion rate of ions in the z-direction and r-direction gradually decreases. Moreover, that decreases faster for the flat microlens array with small opening diameter. As the ion-exchanging time increases, the focal lengths of the microlens arrays with two apertures gradually become shorter, and the focal lengths of the smaller aperture diameters are relatively shorter. The numerical aperture of the planar microlens arrays with different aperture diameters increases while the ion-exchange time increases, and its distortion decreases with the increase of ion exchange time. The rule that the optical characteristics of the gradient index planar microlens array changes with the ion-exchange time provides a reference for the production of the planar microlens array required in different optical systems.