A stereolithography 3D printing method for multiple light modules includes following steps of: controlling a stereolithography 3D printer to retrieve a plurality of slice images and offsets respectively corresponding to different layers of 3D object data; selecting one of the slice images corresponding to one of the layers; adjusting an irradiation range of each of light modules according to one of the offsets corresponding to the same layer; the adjusted irradiation ranges of the light modules don't overlap with each other in a horizontal axis direction; controlling each light module to irradiate according to the adjusted irradiation range and the selected slice image; and, repeatedly executing aforementioned steps to generate a physical 3D model. Therefore, effectively implementing large size stereolithography 3D printing and manufacturing the physical 3D model without any obvious borderline can be achieved.