In order to measure quantum yield (quantum efficiency) of light in the research of fluorescence excitation, rapid prototyping of micro-structure and photochemical, the absorption spectrum must be precisely measured. Therefore, the reflection on optical media interface, absorption and transmission in the medium must also be considered. In this article, we exhibit a set of reasonable experimental method which is designed to measure absorption spectrum by grating spectrometer. The results show that the obtained absorption lines are properly reliable if we select an appropriate way to settle the sample. Furthermore, it can be well generalized and applied to the selection of fluorescence excitation spectrum, photochemical reaction spectrum, photoelectric device spectrum.