A novel method is presented to acquire the demodulation coefficients of the intensity-modulated spectropolarimeter (IMS) by measuring the reference light, for the demodulation coefficients obtained by theoretical calculation cannot demodulate the measured data. The demodulation coefficients of IMS can be separated from the intensity spectrum of the reference light passing through the modulator of the IMS, and it is necessary to process the intensity spectrum, such as IFFT, filtering and FFT etc. The principle of the method is analysed in details and the measured results of the demodulation coefficients for IMS prototype are presented. The measured data of quasi-natural light and polarized light from IMS prototype is demodulated by the demodulation coefficients obtained to verify their validity. Experimental results show that the polarization degree of the light from a tungsten-halogen lamp collimator directly is less than 10%, but it is about 100% when the light is polarized by a polarizer in the spectral range of 550~650 nm. These results are coincident with the theoretical results well, which verify the feasibility of the method presented before.