The molecule structures of Ponceau 4R in ground state and the excited state were optimized by employing the Gaussian 09W program package. In addition, the electronic structure and frontier orbital of the ground state, the emission wavelength of the excited state was also investigated. And then, the Edinburgh FLS920P fluorescence spectrometer was applied to the measurement of the fluorescence spectra of cochineal solution, and the emission spectra was obtained. The calculated emission wavelength had a good coincidence with the experiment data, which indicates that the optimized structures mentioned above are reasonable. The structures comparison between the ground state and the excited state was also performed to analyze the mechanism of fluorescence spectrum. It can be concluded that the molecule structure of excited state is nearly planar, so Ponceau 4R is thought to have strong fluorescent characteristics, the emission fluorescence is the result of transition from orbit 139 to orbit 137.