The contributions of explicit water molecules to color-tuning mechanism of firefly were studied. The explicit water molecules cause two different structures in the geometrical parameters of keto（-1） both in vacuo and aqueous solution. There are somewhat larger influences on absorption and emission spectra. When water molecules were added only on the side of benzothiazole ring, the spectra shift to the blue. In contrast, when waters were added only on the side of the thiazoline ring, the spectra shift to red. In a word, the color modulation of the emitted light depends on charge redistribution of molecule keto（-1）, mainly the charge change of the benzothiazole and thiazole tings at the two terminal in keto（-1）.
