With the fast developments of the technologies of the modern communication and relay protection ,RCS-931B line relay protection device is used widely in the super-high voltage line relay protection.By means of the analysis of the principle of the RCS-931B fibre-optical electric current differential protection,and the analysis of the trip relays of RCS-931B line relay protection device,and the analysis of the application of RCS-931B line relay protection device in the DaDa Power Plant,the paper brings out several questions in engineering application ,and finds out the effective solutions to the questions.