The design principle and structure of the wireless data transmission devices used for Power transformer on-line calibration was described in this paper, design ideas and processes of the low-power hardware and software was introduced in detail. A prototype of the low-power and high anti-interference wireless data transmission device was developed, and a field test was conducted in the substation. The experiments showed that the wireless data transmission device can transmit the experimental data steadily, and update the device program via wirelessly, reaching the purposes of calibrate the transformer online.