An automatic coloring device for moving coloring tool along a curve includes a guide-bar, a moving platform, a coloring tool, a first moving member and a second moving member. The moving platform is movably disposed on the guide-bar, and the coloring tool is disposed on the moving platform. The first moving member controls the guide-bar to move along a curve, and the second moving member controls the moving platform to move along the guide-bar. An axle center of the guide-bar penetrates the plane where the curve is disposed.