The derivative spectrum technique has been widely applied to spectral analysis due to its high resolution and sensitivity. A new kind of derivative spectrum technique is presented in this paper. In order to realize derivative spectrum, a wavelength-intensity splitter has been designed to obtain two light beams with slim difference in wavelength. The two light beams were complementarily modulated with an optical complementary chopper. The modulated light beams were focalized to a photoelectric detector and realized intensity difference. By scanning the wavelength, the derivative spectrum was obtained. The derivative spectrum signal from the detector was acquired and recorded by a computer. Through a lot of experiments, the rationality of the system was approved. The emission spectrum and its first derivative spectrum of the xenon lamp were measured on this system. Finally, the result shows that the first derivative spectrum sensitivity and resolution are both higher than that of the emission spectrum.