Different discharge modes in different boundary discharge domains at the same experimental condition are observed in argon/air mixture in a dielectric barrier discharge system with two large diameter water electrodes. Regular patterns and random filaments are formed in the closed square boundary and the semiopen domain respectively. It is found that the relatively intensity of the several higher excitation energy spectral lines such as 696.5, 727.3, 750.4 and 772.4 nm increases with the applied voltage in the closed boundary domain while decreases in the semiopen domain. Results show that the electron average energy in the closed boundary is higher than that in the semiopen domain and the difference of the electron average energy increases with the applied voltage. The results of molecular vibration temperature estimated by the second positive spectrum of N2 molecular indicate that the vibration temperature increases with the applied voltage in the closed boundary and decreases with the applied voltage in the semiopen boundary domain.