The effects of the central wavelength shift of the source and the variations of the parameters of the sensing head(Verdet constant and linear birefringence) due to ambient temperature fluctuation upon the scale factor of a bulk-glass optical current transformer are theoretically analyzed and digitally simulated. The results show that the normalized output scale factor will decrease 1.379%,when the ambient temperature changes from 10℃ to 30℃,among which the effect of the central wavelength shift with temperature upon the scale factor is greater than the others. Therefore,it is necessary to adopt proper technical measures to decrease or compensate the undesirable effects of the temperature characteristics of the sensing head.