Abstract: Gas relay is the main non-electrical protection device of transformer,which plays an important role. Traditional detection methods can not reflect the gas relay level,and there will be gas relay false alarm in operation and maintenance.Firstly,the working principle of gas relay was analyzed,and a non-conductive capacitance sensor was proposed to realize the real-time monitoring of gas volume in gas relay.Then,through the selection analysis and optimization design of gas protection module and liquid level measurement module,a new on-line monitoring scheme of intelligent gas relay was constructed.Based on the change state of gas volume in the gas relay,more accurate and timely fault diagnosis of power transformer was realized,and the prototype processing was completed.Finally,the feasibility of the volumetric testing method was verified by comparing the two conditions of different liquid level heights and different ambient temperatures.The proposed method is of great significance for discovering the latent faults of transformer,and realizing the condition monitoring of oil immersed power transformer,which ensures the safety of power grid equipment and personnel.