Cold plasma generated by atmospheric air discharge has wide application prospect in industry because it does not need vacuum equipment and mass production is possible. In this paper, a stable uniform discharge is generated in open air by a plasma needle. Discharge mechanism is investigated by optical method, and plasma parameters are given by the spatially resolved measurement of emission spectrum from the discharge. Results show that the discharges have two modes. One is a corona discharge mode and the other is plasma plume mode. In the stable plasma plume mode, a strong emission area and a weak emission one can be distinguished from each other. The development velocity of the weak emission area is much faster than that of the strong emission area. Furthermore, the electron energy and the plasma density in the weak emission area are also bigger than those in the strong emission area. Therefore, the discharge in the strong emission area is dominated by Townsend mechanism, while that in the weak emission area is dominated by streamer discharge. Gas temperature and vibration temperature are also studied in this paper. The experimental results are of great importance to the industrial applications of atmospheric pressure discharge.