Based on the fundamental results on MDS self-dual codes over finite fields constructed via generalized Reed-Solomon codes [9] and extended generalized Reed-Solomon codes [15], many series of MDS self-dual codes with different length have been obtained recently by a variety of constructions and individual computations. In this paper, we present an unified approach to get several previous results with concise statements and simplified proofs, and some new constructions on MDS self-dual codes. In the conclusion section we raise two open problems.2010 Mathematics Subject Classification. 11T06, 11T55.