The invention provides a simple high-temperature testing device and method for a flange rubber sealing element. A lower flange is in butt joint with an upper flange; the end face of the lower flange which is contacted with the upper flange is provided with a ring groove which serves a sealing groove for assembling the flange rubber sealing element to be tested; an upper cover is welded on the top of the upper flange; a bottom cover is welded at the bottom of the lower flange; the upper cover is connected to a gas medium charging pipeline; one end of a thermocouple is inserted into the upper flange through a gap, and the other end of the thermocouple is connected with a temperature indicator; and the upper flange and the lower flange which are connected one above the other are arranged in a high-temperature oil bath container. In the invention, a gas source is cut off by pressurizing under the normal-temperature condition, and a pressurizing way for reaching a preset pressure at the high temperature by using a gas expanding principle is adopted, so that the pressure constancy under the high-temperature condition is ensured; and an oil bath heating way is adopted, so that rapid heat transfer of a leakage test tool is realized. Due to the adoption of the device and the method, detection of the high-temperature sealing property of the rubber sealing element at the extreme usage temperature during short-time use is realized, and the development cost is greatly saved.