A quick coloring method for a 3D printer (1) having an ink nozzle-head (13) includes the following steps: reading an image file and a coloring route; controlling the ink nozzle-head (13) to move to a start position (51) of one of printing rows; controlling the ink nozzle-head (13) to move and to print a coloring object (3) according to the image file and the coloring route; reading a reverse image file and a reverse coloring route, in which the reverse image file records a horizontal reverse image of the image file, and the reverse coloring route records an opposite start position (61); controlling the ink nozzle-head (13) to move to the opposite start position (61) of next printing row according to the reverse coloring route; controlling the ink nozzle-head (13) to move and to print the coloring object (3) according to the reverse image file and the reverse coloring route; and re-executing above steps before a coloring operation is completed.