The diffuse discharge plasma in air was observed in a dielectric barrier discharge with two semispherical water electrodes. The variations of vibration temperature, rotation temperature, and average electron energy as the function of the applied voltage were studied by emission spectroscopy. The vibration temperature and the rotation temperature were calculated through the second positive band system (Ｃ　３Πｕ→Ｂ　３Πｇ) of N+2 and the first negative band system (Ｂ　２Σ＋ｕ→Ｘ　２Σ＋ｇ) of N+2 respectively. The average electron energy was studied by intensity ratio of 391.4 and 337.1 nm. It was found that the rotation temperature increases with the applied voltage increasing, while the vibration temperature and the electron energy decrease.