It is well known that low bit rate BDCT coded image exhibits visually annoying coding artifacts. In this paper, we proposed a POCS (projection onto convex sets)-based deblocking algorithm using a novel region smoothness constraint for the class of graphic images. For this type of image, there usually exists large smooth varying regions. Current POCS based deblocking algorithms usually do not enforce smoothness across several blocks. In our method, the BDCT (block-based discrete cosine transform) coded image is modeled using a smooth, spatially adaptive, thin-plate spline surface, and the region smoothness constraint set is constructed from this spline surface. The spline modeling allows us to model the long range smoothness across the smooth region. Simulation experiments indicated that blockiness in smooth regions is effectively suppressed while genuine edges are preserved.