A cold atmospheric plasma (CAP) device has been developed to assess the bacterial inactivation on an aqueous brass surface for clinical trials. The Miles and Misra method and the decimal dilution method are used to count the number of colony forming units for mutual verification to certify the validity of this investigation. The experimental results show an exciting much higher bacterial inactivation on this CAP-brass system compared to the traditional sole CAP treatment; about 6–7 log reduction can be achieved in 30–60 s of application. This system can be used as a standard target to test the efficacy of plasma treatment in an aqueous condition, especially in hygiene, medicine, and food processing applications.