A small suitable digital all-optical current measurement system is studied which is presented under the principle of the optic-fiber current sensors, is used by the advantage of the strong anti-interference ability and the complete electrical isolation, aimed to solve the actual technical problems of the existing power system high voltage side current detection system, such as the strong electromagnetic interference, the complicated insulation system and the high cost of the maintenance and so on. This paper will improve the traditional electromagnetic current transformer in the power system. It used polarization maintaining fiber to inhibit the inherent linear birefringence in the fiber effectively, used Jones matrix to analysis of the fiber-optic system and used MATLAB simulation software to simulate the system. The theoretical analysis and system simulation have verified that the digital all-fiber current transformer measured current and the actual current is consistent. Shows that the system established mathematical model and simulation model is correct and feasible.