The present invention provides a method and system for printing a 3D object. The method for printing a 3D object includes the following steps: a. performing conversion based on layered image data DM of a target object to obtain printing data PMN, where DM consists of a value set XMN of NM pixel points, M represents the number of layers of a layered image, and N represents the number of pixel points; b. performing layer-by-layer printing based on the printing data PMN, where when PMN is 0, a first printing material is used for printing, and when the printing data PMN is 1, a second printing material is used for printing; and c. superposing results of the layer-by-layer printing in step b to form a 3D object.