A method for determining AlGaInP-based LED junction temperature was developed based on the relative spectral difference. The relative spectra of different LED with various drive currents and substrate temperatures were measured by spectrometer, and the differences of LED relative spectra were calculated. And then the linearity of the relative spectral differences and junction temperatures was analyzed. At last, the acurracy of the proposed method was compared with the peak wavelength method. The results show good positive correlation between relative spectral difference and junction temperatue. The acurrucy and the error of the proposed method become worse as the bandwith of spectrometer increases. The acurrucy of proposed method with the spectrometer of 2 nm bandwith is much higher than the peak wavelenght method of 1 nm bandwith. While the propose method of 5 nm bandwith has the same acurrucy with the peak wavelenght method of 1 nm bandwith.