To synthesize three different grafting ratios of gallic acid (GA)-chitosan (CS) copolymer by a free radical mediated grafting method, which is further applied to the field of antibacterial materials, crosslinking structures of the compound GA-CS copolymer were characterized, fully indicating that gallic acid is resoundingly grafted onto chitosan. The grafting ratios of three copolymers GA-CS are 45.71% (Ⅰ), 36.12% (Ⅱ), and 18.96% (Ⅲ) were determined by UV-Vis spectrophotometer. The minimum inhibitory concentrations of three GA-CS copolymers are 30 μg/mL against Escherichia coli and are ranged from 250 to 550 μg/mL against Staphylococcus aureus. By counting viable bacterial colonies, it can be found that antibacterial property is preferable by increasing the grafting ratio of GA-CS copolymers. Findings of investigation on aforementioned bacteria experiment indicate that the CFU/mL values of GA-CS (Ⅰ, Ⅱ, Ⅲ) are 2.04×106, 7.56×106, 1.48×107 to Staphylococcus aureus, and 2.96×106, 1.01×107, 2.14×107 to Escherichia coli after 12 h treatment. In addition, the interaction process between GA-CS copolymer and bacteria can be observed through a transmission electron microscope. The specific manifestation is that the bacterial cell membranes are ruptured after being treated with the copolymer, which causes the cell contents to flow out, and the cell morphology is shrunk and out of shape.