Method and apparatus for constrained de-blocking filter are disclosed. According to one method, a current block is partitioned into a plurality of sub-blocks using SDIP (Short Distance Intra Prediction mod). A first Bs (boundary strength) for an internal block boundary of the plurality of sub-blocks is determined by setting the first Bs to a second Bs of an Intra-coded boundary block of the current block. De-blocking process is applied, using the first Bs, to reconstructed samples across the internal block boundary of the plurality of sub-blocks to generate filtered-reconstructed samples. In another method, the current block is partitioned into two sub-blocks using SBT (sub-block transform) horizontally or vertically and the first Bs (boundary strength) is determined for an internal block boundary between the two sub-blocks by setting the first Bs to a second Bs of a non-zero cbf (coded block flag) block of the two sub-blocks in step.