1. A three-dimensional printing system, comprising: a light source unit, configured to illuminate an object to be measured; at least one image-capturing module, comprising: an image-capturing unit, disposed at a side of the object to be measured; and a focus-adjusting lens group, disposed between the image-capturing unit and the object to be measured; a processing unit, controlling the focus-adjusting lens group to change shooting focal lengths and controlling the image-capturing unit to capture a plurality of images of the object to be measured under the different shooting focal lengths, wherein each of the plural images comprises a focused local image and an unfocused local image therein, and the processing unit calculates out a three-dimensional profile data of the object to be measured according to the focused local images in the images and the shooting focal lengths corresponding to the focused local images; and a printing device, printing a plurality of cross-sectional profiles corresponding to the object to be measured.