1. A three-dimensional printing apparatus, comprising: a processing unit, configured to read and process a digital three-dimensional model information; a base, having a carrying surface; a three-dimensional display unit, coupled to and controlled by the processing unit and configured to display a digital three-dimensional image related to the digital three-dimensional model information on the carrying surface for previewing, wherein the three-dimensional image is corresponding to a three-dimensional object related to the digital three-dimensional model information; and a printing head, disposed above the base and coupled to the processing unit, wherein the processing unit controls the printing head to dispense a base material on the carrying surface layer by layer along an image contour of the three-dimensional image so as to form the three-dimensional object according to the digital three-dimensional model information.