Proper transform of Current Transformer (CT) is the precondition for the proper operation of protective relays. However, the field test data can not be directly used to predict the excitation current when large short circuit current, it’s hard to make a quantitative evaluation or exact calculation of CT error. A comprehensive analytic expression of three spline interpolation and exponential function is proposed. It uses a subsection processing to fit I-V character curve, which can completely describe the linear and saturation regions of I-V character and obtain the excitation voltage and current value of the saturated region, thus it can calculate the transmission error and the maximum allowable load of the current transformer when short circuit fault occurs. After verification with the measured data, the measured data can be well fitted and the calculated results are accurate. Results show it has a good engineering application value.