With the increasing degree of automation of the power system, the utilization rate of electronic transformer is also increasing. However, the instability of the electronic transformer and the influence of the external environmental factors will lead to the measurement error. The main content of this paper is the design of electronic transformer on-line error comparison system, including the establishment and on line software joint debugging and operation of Primary system, secondary system, this system analysis on the factors of transformer ratio difference. In this study, the research of electronic transformer technology is used to accumulate the experimental data, which provides a powerful support for the development of electronic transformer.