Abstract: LED junction temperature is an important indicator of its performance and is relevant to its thermal resistance. It impacts on LED life. In this paper, the fourthermal resistance theory of LED is proposed and analyzed by measuring the voltage, current and heat sink temperature of high-power LED lamp net based on the good linear relationship between junction voltage and junction temperature of LED semiconductor device. An automatic measuring instrument was developed for high-power LED light grid thermal resistance. It can perform the automatic testing of the high-power LED light network thermal resistance quickly and accurately.