The intensity of broadband illuminant fluctuates when its power supply output power changes. Spectral intensities at each wavelength within the band of broadband illuminant fluctuate at different levels. A method based on spectrum linear fitting is proposed to compensate the illuminant spectral intensity in its band when its intensity fluctuates. The spectral intensity fluctuation at each wavelength could be compensated simply by measuring the band intensity with this method. The linear relationship between spectral radiant exitance and whole radiant exitance of ideal blackbody was analysed by researching the radiant exitance at different temperatures. The linear model of broadband illuminant band intensity and spectral intensity was built. Experimental system is composed of a halogen light, a power supply, an aperture, a spectrometer, and a computer mainly. By adjusting the power output of the power supply, we obtained a set of halogen light relative spectral intensities at different power inputs. The spectral intensity of halogen light at different input powers was measured to test the compensation effect of this method. The relationship between spectral intensity and band intensity of halogen light was fitted with linear relation and the fitting errors were analysed. The experimental result shows a linear relationship between spectral intensity and band intensity of halogen light, so the spectral intensity fluctuation can be compensated using the band intensity according to their linear relation. The relative error absolute value of compensated spectral intensity decreases as the halogen light input power increases. Within the range of halogen light input power, the relative error absolute values of spectral intensity compensated with this method are within 5% at vast majority (92%) of the wavelengths.