To eliminate the influence of light source fluctuation in an optical object chromaticity measurement system, this study proposes a new algorithm for object chroma based on quadratic normalized coefficient and develope a measuring device. Based on the principle of no change in the original optical path, the device is installed at the back of a monochromator. The light from light source is divided into two parts: measurement beam and monitoring beam. Consequently, the measuring beam is normalized to the monitoring beam. Results obtains by changing the output current of the driving power to simulate the fluctuation of the light source using the proposed and conventional methods are compared. Experimental results show that the relative error of the proposed method is no more than 0.51%. Thus, the proposed method can effectively improve the stability and accuracy of the measurement system and eliminate the influence of the light source fluctuation.