Abstract: For a long time, power transformers are usually equipped with gas relay protections, which work with current differential protection as the transformer main protection. However, due to the lack of theoretical innovation and performance improvement, the deficiencies of reliability and speed remain in the existing gas relay protections. Therefore, this paper presented a theoretical model and its simulation method for the research on the relationship between the electrical faults and non-electric characteristics inside transformer tank. According to the huge differences among the internal faults, external faults and other running status, a novel transformer digital non-electric protection principle based on the pressure characteristics has been proposed. Simulation test results verify that this protection principle is able to reliably and quickly clear the internal faults, even a weak fault, without inrush current influence.