The fluorescence spectra of ethanol-water solution excited by UV-light have been studied, in which there is a broad and featureless fluorescence spectrum with the peak at 410 nm. It is considered that when the clustered molecules with a special long chain structure approach each other, the excimer is formed between the excited state and ground state cluster monomers, which can emit fluorescence. According to the experimental result and the energy transfer theory, we know that an electron transfer complex is formed by the excimer and the cluster monomer in the excited state. Based on Mulliken theory and the quantum mechanics theory, the energy of ground state EbN and excited state ＥbE, as well as the electrostatic interaction energy Ｅs which is caused by electron migration from donor to acceptor, have been calculated, respectively. Based on the theory of one-dimensional harmonic vibrator model, the energy valley in the potential energy curve when the excimer is formed has been simplified. Thus the potential energy function and the energy value of the excimer formed by the clusters in the critical distance can be calculated, and then the electron migration electrostatic interaction energy can be gotten too. Accordingly the fluorescence emission mechanism of the broad and featureless fluorescence peak has been explained theoretically.