The work aims to study the simple and quick methods used to prepare graphene/silver nanoparticle (AgNP/G) composite antimicrobial materials. AgNP/G nanocomposite was prepared in the alkaline environment in in-situ reduction method. The structure and morphology of AgNP/G composites were characterized by X-ray diffraction, infrared, ultraviolet and transmission electron microscopy. The formation mechanism of AgNP/G composites was investigated. The antimicrobial properties of AgNP/G composites were observed by plate counting method. In the prepared AgNP/G composite, the nano-silver formed was small in size (15 nm), uniform in particle size, and evenly distributed on the graphene sheet. When the antimicrobial concentration of AgNP/G was 20 μg/mL, the antibacterial rate could reach 98.7%. The presence of alkali can accelerate the formation of silver nanoparticles on the graphene sheet, and the prepared AgNP/G composite has excellent antibacterial properties.