The invention discloses an on-line monitoring method of internal temperature of SF6 high-voltage electrical equipment. The on-line monitoring method of the internal temperature of the SF6 high-voltage electrical equipment comprises the steps that a first pressure value and a first temperature value of gas inside the SF6 high-voltage electrical equipment are measured, and a density value of the gas inside the SF6 high-voltage electrical equipment is obtained according to the first pressure value and the first temperature value; a second pressure value of the gas inside the SF6 high-voltage electrical equipment is measured, and the internal temperature of the SF6 high-voltage electrical equipment is obtained according to the second pressure value and the density value of the gas inside the SF6 high-voltage electrical equipment. The accurate density of the gas inside the SF6 high-voltage electrical equipment is obtained in time through continuous monitoring and systematic correction, data processing is carried out by utilizing the pressure-temperature characteristic relation of SF6 gas, and a corresponding internal temperature value of the SF6 high-voltage electrical equipment is obtained. The invention further provides an on-line monitoring device and system of the internal temperature of the SF6 high-voltage electrical equipment. The on-line monitoring device and system of the internal temperature of the SF6 high-voltage electrical equipment achieve real-time and accurate measurement of the internal temperature of the electrical equipment, lower detection cost and are suitable for engineering application.