The development of modern smart grid requires much more flexibility and reliability of relay protection system. Problems existing in present relay protection system structure are analyzed combining with the characteristics of smart grid， and the suggestion that making relay protection reconfigurable in future smart grid is proposed. In addition， analysis of the demand of relay protection system on-line reconstruction is carried out. The implication and criterion of relay protection reconstruction and the general model of implementing reconstruction are put forward. Examples of reconstruction are given and the problems to be studied in future are also pointed out.