This standard specifies a test method for determining the minimum oxygen concentration required to sustain combustion of plastics and other materials in high-temperature environments ranging from 25°C to 150°C (typically 40°C to 150°C). The core test parameter is defined as the temperature index at a specific temperature. This method is applicable to rigid or foam materials in the form of strips or sheets that can be self-supporting, as well as flexible sheets or films that can be vertically supported within this temperature range. It provides a critical basis for evaluating the flame-retardant properties of materials under actual high-temperature conditions, such as electrical overheating. The core of this method lies in the use of a high-temperature oxygen index tester. Unlike standard oxygen index testers, the key feature of this instrument is its combustion chamber, which is equipped with heating elements and a preheater to precisely control and maintain the set test temperature inside the chamber. The instrument adjusts the oxygen-nitrogen mixture ratio through high-precision sensors and flow control systems to determine the oxygen index of materials at elevated temperatures.