Tian Ning,Wang Dongsheng,Huang Lei,Gong MaliAiming at solving special requirements of fiber device suited for fiber laser under high power density operation, a new measurement system was designed to meet these special requirements. After comprehensive analysis and comparison with truncation method, backward scattering method and insertion loss method, a new dual optical path measurement system of fiber was proposed. The system can measure the insertion loss of tested fiber and reference fiber at the same time to monitor and calibrate the input optical signal power, so it can achieve higher accuracy. Theoretical consequence and explanation for the measuring method and parameters in this new system are discussed in this paper. The transmission efficiency and measuring accuracy of the system is analyzed, which meet the need of design (Insertion loss ≤0.5 dB, return loss≥50 dB).