Abstract: The non-equilibrium plasmas produced by diffuse discharges have a great potential of application in many high technology fields. In room-temperature atmospheric air, the formation mechanism of non-equilibrium plasma is discussed and analysed. It is concluded that generating diffuse discharge in open air should meet the three conditions: low-voltage excitation, plentiful electron avalanches and temperature inhibition of spatial charge particles. A method of generating diffuse discharge is proposed and implemented. Based on runaway electrons breakdown theory, a low duty-ratio, high voltage repetitive nanosecond pulse generator is chosen as discharge excitation source. Using the wire-electrodes with small curvature radius, the gaps with highly non-uniform electrical field are structured. The experiments are performed in linear-type and ring-type electrode pairs. The results prove that the proposed method can generate typical diffuse discharges in cm. gaps via nanosecond pluses with less than 100 kV peak voltage, hundreds of Hz repetitive frequency.