The invention provides a device and a method for measuring an inner volume of SF6 (sulfur hexafluoride) gas in SF6 gas insulated electrical equipment by means of on-site operation. The device comprises an air cylinder, an inflating valve, a pressure gauge and a check valve, the inflating valve is connected to the pressure gauge through a second connection pipeline, the pressure gauge is connected to the check valve through a first connection pipeline, an air outlet of the air cylinder is connected to the second connection pipeline between the inflating valve and the pressure gauge, the air cylinder is connected with a rotary disc, a volume of the air cylinder is adjusted through rotation of the rotary disc, the inflating valve is connected with newly-bottled SP6 gas , and the check valve is connected with an inflating opening of the measured SF6 gas insulated electrical equipment. The device and the method have the advantages that simplicity in operating steps is realized, and the inner volume of the SF6 gas in the measured SF6 gas insulated electrical equipment can be calculated accurately on the premise that normal operation of the measured equipment is not affected.