The utility model discloses an automatic change digit relay protection of transformer substation tester, including a central processing unit and respectively connected to the time unit, light yield input unit and little signal output unit, a central processing unit be used for with the communication of industrial computer and light numerical portion input/output, with CPU synchronization and communication, to the time and little signal output, the utility model discloses can character recognition protection device, measurement and control device, intelligent terminal's definite value, can inspect protection device, measurement and control device, intelligent terminal communication uniformity.