Abstract: To generate energy-efficient, reactive, homogeneous, and stable atmospheric pressure plasma sources, Ar dielectric barrier discharge(DBD) was excited by nanosecond pulsed power supply, and H₂O was added to enhance plasma reactivity. The discharge characteristics of the nanosecond pulsed Ar DBD with H₂O addition were investigated by electrical and optical diagnoses. The discharge uniformity was quantitatively calculated by the Gray Value Standard Deviation method, and the electrical parameters were studied by an equivalent electrical model. The results show that the ns-pulsed Ar DBD with 0%~0.2% H₂O content is in a uniform state. When the H₂O content is increased further, the added H₂O absorbs electrons and distorts the space electric field, resulting in bright filaments in space and the decrease of discharge uniformity. And the excessive H₂O will render the discharge to be extinguished. Since the addition of a small amount of H₂O can promote the Penning ionization process in the plasma, the transported charges, average discharge power and energy efficiency increase with the increase of H₂O content first, reach a maximum value at 0.1% H₂O content, and finally decrease with the increase of H₂O content. The plasma reactivity is characterized by the emission spectrum intensities of OH and Ar lines. It is found that, at 0.1% H₂O content, the intensities of OH and Ar lines reach the maximum values. The ratio of emission intensities of Ar lines indicates that the electron energy increases with the increase of H₂O content, reaches a maximum value at 0.1% H₂O content, and then drops.