This paper introduces in detail the detection requirements for on-line monitoring of gas in transformer oil, focuses on the types of gas detection methods in transformer oil and transformer fault diagnosis technology, and analyzes the advantages, disadvantages and development status of horizontal technology comparison and application comparison. It provides a reference for the substation to select the appropriate online monitoring equipment or related research, and is of great significance to improve the accuracy of transformer operation status evaluation and fault diagnosis, and maintain the safe and stable operation of the power grid.