Cold atmospheric plasma (CAP) is an effective new antimicrobial approach that is gaining increasing attention and has a wide range of potential applications in biomedical fields. Among all of the bactericidal factors generated by CAP, the synergy of reactive nitrogen species (RNS) and reactive oxygen species is generally considered as the main reason for its high bactericidal efficiency. However, the produced RNS (such as nitrite) may also pose potential risks to human health. Therefore, it is of significance to keep the high disinfection efficiency of CAP but with producing no or little harmful RNS. In this study, we investigated whether it is possible to improve the disinfection efficiency of CAP without producing the harmful RNS by adding a certain amount of inert halogen salt such as potassium iodide (KI). We found that the inactivation of both Gram-negative and Gram-positive bacteria by helium atmospheric pressure plasma jet (He-APPJ), one form of CAP, is enhanced consistently in the presence of a certain amount of KI. The mechanism of action is due to the fact that the He-APPJ-generated hydrogen peroxide (H2O2) oxidizes the iodide anion to triiodide (I3 -), which contributes to the major bactericidal activity. We believe that the results in this work can be highly relevant to the practical application of plasma for disinfection in the biomedical field.