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, wherein the operation that the control unit compares the object image with the checking image comprises: comparing profile information of the object image with profile information of the checking image.