Drinking Water safety is critical for human's daily life. Real-time monitoring and early warning systems of harmful substances in drinking water is crucial to ensure the safety of tap water. This paper reports the fabrication of a graphene-based phototransistor, oriented towards an integrated system for high accuracy measure of the bioluminescent bacteria. Aliivibrio fischeri is a bioluminescence bacterium (light emission at 490nm), that has a significant light reduction in the presence of harmful contaminants. Each step of the phototransistor was designed and fabricated, including the mask used for the Oxygen Plasma Etching of Graphene and Electron Beam deposition of the gold pads and Spin coating of the polymer. All the fabrication process including experimental conditions were controlled to achieve a high phototransistor performance. Experimental tests were realized to evaluate the performance of the photodetector to the measurement of Vibrio Fischeri light emission The graphene-based phototransistor shows good sensitivity to detect the change of light intensity, for Aliivibrio fischeri. Compared with the traditional design method, this new design and fabrication can not only be more distinct and visualized, but also greatly reduce the cost and difficulty.