The error of current transformer is not constant, which is related to secondary burden. Verification certificate shows the error data of current transformer at rated burden and lower limit burden. Therefore, error at specific burden is unavailable. This paper presents error calculation method at any burden under the condition of known error data at rated burden and lower limit burden. Meanwhile, it is verified through experiment. The result shows that the error between calculated value and measured value is less than two rounding intervals at class 0.2. Thus, calculation method of error at any burden deduced in this paper can be applied in the practical work.