Substandard ratio error under full temperature(-40-60 ℃) is one of the factors that restrains the practical application of domestic All Fiber Optical Current Transformer(FOCT). The Verdet constant of the sensing fiber changes with the fluctuation of mean wavelength when the light goes through. Then the output ratio is influenced which decreases the accuracy of the system. The relation between the fluctuation of mean wavelength and system ratio error was analyzed thoroughly and validated experimentally. Also, influence on the mean wavelength caused by the entire optical components was theoretically analyzed, and the degree of influence and the variation trend were experimentally measured. A method was put forward to make the polarizer and the fiber delay line have a temperature self-compensation on the variation of mean wavelength, and thus the change of mean wavelength was reduced. While the 0.2 s class accuracy requirement is guaranteed, the impact can be lowered as much as possible.