Abstract: The polarity detection of transformers is an important task to ensure the accuracy of electrical energy measurement. This article designs a new type of transformer polarity detection instrument for on-site measurement, with the main structure including the main operating rod, control circuit, and testing circuit. The detector can prevent workers from directly entering the measuring cabinet for work, and the lighting on the top of the instrument makes it easy for workers to see the equipment inside the cabinet, enhancing work safety; The voltage decay rate and cycle delay rate test performance of the output signal meet the requirements. The polarity detector of this transformer has important application value for improving the safety and accuracy of electric energy metering.