The transition from filamentary discharge to uniform discharge in a device with water electrodes is studied in air by electrical method and emission spectroscopy. Results show that discharge can transit from filamentary mode to uniform mode by increasing the applied voltage or decreasing the gas pressure. The discharge mechanism at high pressure is the streamer and that at low pressure is glow discharge. The relations of the number of high-energy electons and experimental parameters were studied by optical emission spectroscopy. The results show that high-energy electrons increases in number with decreasing gas pressure and decreases with increasing applied voltage. The experimental results are explained qualitatively based on the theory of wall charge. The results are of value for the study of uniform discharge in dielectric barrier discharge.