According to the luminous spectrum characteristics of white light emission diode (WLED) light emission spectrum, through the analysis of the intersection (the trough point of the whole spectrum) of blue light spectrum and yellow light spectrum generated by blue light excited yellow phosphor, in this paper we design an LED steady-state thermal resistance measurement system based on the spectroscopic method by using the conventional spectrometer, and we also use the normal driving current to fit the whole spectrum trough through a certain function algorithm. According to the temperature rise curve, we can calculate the temperature rise of the LED junction temperature relative to the substrate under any working condition, and combine the heat dissipation power of the LED to get the steady-state thermal resistance of the LED. This method avoids the limitation of a similar forward voltage drop method which uses the minimum current calibration and requires the modules of high-speed data acquisition and high-speed sampling conversion, thus making the equipment expensive. Therefore it is necessary to reduce its cost. Finally, the system designed in this paper and the T3Ster instrument of Mentor Graphics Corporation in the United States are both used to measure various LEDs and their results are compared with each other. The results show that the maximum deviation of steady-state thermal resistance is only 3.64%. It indicates that the system and method designed in this paper can achieve the same precision as T3Ster instrument of Mentor Graphics Corporation, demonstrating that the system and method designed in this paper can achieve the same precision as the T3Ster instrument of Mentor Graphics, under the condition without needing expensive equipment, Moreover, this method uses non-traditional spectral method to measure the junction temperature of LED, which has the characteristics of remote real-time online detection of LED junction temperature, low cost, and no restrictions on the LED packaging structure. Therefore, this method has a wider application range than the voltage method adopted by Mentor Graphics T3Ster equipment, and has a certain practical value.