The question whether the emitter of yellow-green firefly bioluminescence is the enol or keto-constrained form of oxyluciferin (OxyLH2) still has no definitive answer from experiment or theory. In this study, Arg220, His247, adenosine monophosphate (AMP), Water324, Phe249, Gly343, and Ser349, which make the dominant contributions to color tuning of the fluorescence, are selected to simulate the luciferase (Luc) environment and thus elucidate the origin of firefly bioluminescence. Their respective and compositive effects on OxyLH2 are considered and the electronic absorption and emission spectra are investigated with B3LYP, B3PW91, and PBE1KCIS methods. Comparing the respective effects in the gas and aqueous phases revealed that the emission transition is prohibited in the gas phase but allowed in the aqueous phase. For the compositive effects, the optimized geometry shows that OxyLH2 exists in the keto(−1) form when Arg220, His247, AMP, Water324, Phe249, Gly343, and Ser349 are all included in the model. Furthermore, the emission maximum wavelength of keto(−1)+Arg+His+AMP+H2O+Phe+Gly+Ser is close to the experimental value (560 nm). We conclude that the keto(−1) form of OxyLH2 is a possible emitter which can produce yellow-green bioluminescence because of the compositive effects of Arg220, His247, AMP, Water324, Phe249, Gly343, and Ser349 in the luciferase environment. Moreover, AMP may be involved in enolization of the keto(−1) form of OxyLH2. Water324 is indispensable with respect to the environmental factors around luciferin (LH2).