The present invention provides a method for performing deblocking filtering, comprising the steps of: identifying a transform unit (TU) boundary and a prediction unit (PU) boundary in the current block; when a neighboring PU adjacent to the current PU includes a boundary, determining an additional PU boundary extended in the boundary; determining boundary strengths (BSs) for the TU boundary, the PU boundary, and the additional PU boundary; and performing a filtering on the basis of the determined BSs.