Abstract: In the power plant, the motor with capacity more than 2000kW should be installed with the differential protection. In order to promote the property of the differential protection and prevent from misaction, and by employing the Indonesia Palabuhanratu 3?350MW coal fired power plant for background, the parametes of current transformer and protection device of long distance differential protection for large capacity medium voltage motor are analyzed. According to principles of the differential protection, through comparing accuracy limit factor, conversion ratio and cable distance from protection device to motor, it is found that choosing small rated secondary current, placing protection and current transformer beside the motor would convert fault current exactly and decrease the sencodary load for saft running, which is confirmd by the actual project.