1. A three-dimensional printing apparatus, comprising: a movable platform, comprising a carrying surface; a printing unit, disposed above the movable platform and forming at least one layer object on the carrying surface based on a three-dimensional model information; an image sensor module, disposed above the movable platform to capture an object image of the at least one layer object in a capturing direction; and a control unit, coupled with the printing unit and the image sensor module, wherein the control unit determines a checking image based on the three-dimensional model information and the capturing direction and compares the object image with the checking image.