Atmospheric pressure helium/water dielectric barrier discharge (DBD) plasma is used to investigate the generation of reactive species in a gas-liquid interface and in a liquid. The emission intensity of the reactive species is measured by optical emission spectroscopy (OES) with different discharge powers at the gas-liquid interface. Spectrophotometry is used to analyze the reactive species induced by the plasma in the liquid. The concentration of OH radicals reaches 2.2 mu m after 3 min of discharge treatment. In addition, the concentration of primary long-lived reactive species such as H2O2, NO3- and O-3 are measured based on plasma treatment time. After 5 min of discharge treatment, the concentration of H2O2, NO3-, and O-3 increased from 0 mg . L-1 to 96 mg . L-1, 19.5 mg . L-1, and 3.5 mg . L-1, respectively. The water treated by plasma still contained a considerable concentration of reactive species after 6 h of storage. The results will contribute to optimizing the DBD plasma system for biological decontamination.