In this work, we seek to understand how the fluorescence spectrum is modulated by the protein environment of bacterial luciferase, especially focusing on the effect of the electrostatic interaction with the fluorophore, HFOH. The effects of the protein environment on the fluorescence spectroscopic properties can be obtained by comparing the spectrum calculated on the fluorophore embedded in the protein environment (System I) to the corresponding spectrum calculated on the bare fluorophore using the vacuum-optimized geometry (System II) and the inprotein structure (System III). Specifically, comparison of the results of the bare fluorophore in Systems II and III gives the indirect effect of the protein surroundings through induced structural changes in the chromophore, while the direct electrostatic contributions can be obtained by the comparison of results between Systems I and III. Besides, we systematically investigate the spectroscopic properties of LFOH (4a-hydroxy-5-hydro-lumiflavin), a simplified model molecule of HFOH, in various solvents and in oriented external electric fields (OEEFs) to get an in-depth understanding of the spectral tuning mechanism in bacterial luciferase. In line with previous studies, the indirect effect, i.e. geometry confinement by the steric effect of the protein environment, is found to be fundamental to its fluorescence efficiency. On the other hand, the electrostatic effect is found to play key roles in the fine-tuning of the spectrum, including emission maximum λ F and intensity. Intriguingly, we find that the electrostatic effect of bacterial luciferase can be attributed to an internal electric field (IEF) pointing toward the negative F X direction (from the pyrimidine ring to the xylene moiety) with a magnitude of ca. 30 MV/cm.