A method for printing a model for a 3D printer includes: perform a slicing process on an outer-frame object for generating multiple slicing-route files of multiple printing layers; perform the slicing process on a modeling object for generating multiple inkjet graphic files of the multiple printing layers; print a slicing object of one printing layer according to corresponding one of the slicing-route files; perform a filling process to the inside of the printed slicing object; perform a coloring process on the inside of the printed slicing object according to one of the inkjet graphic files corresponding to the same printing layer; and re-execute the above printing steps until all the printing layers are printed. The inside of the 3D model is colored via the inkjet graphic files generated after processing on the modeling object generating the visual effect that the modeling object is accommodated within the 3D model.