As the output end of the high power fiber laser resonator, the reflectivity value of the output coupler(OC) was small. When the OC reflectivity was measured by the transmission spectrum method, the instability of the light source power or the error form measured spectrometer can cause a large error in the result of OC′s reflectivity. Based on Fresnel reflection principle, a method of measuring OC reflectivity by reflectance spectrum was proposed. Validating the method by experiments, the results show that the measurement accuracy and stability of the method have been improved remarkably. At the same time, the inaccuracy of bandwidth measurement caused by this method was analyzed, and a method of using index matching liquid was proposed to solve the problem.