Atmospheric pressure microwave plasmas have presented significant application values in decomposing and converting carbon dioxide to treat the environment due to their unique advantages such as high density of active particles, high gas temperature, high energy conversion efficiency, and good controllability. In the work, the discharge characteristics and the temperature parameters of CO2 microwave plasma were studied and diagnosed to apply decomposition and conversion of carbon dioxide with atmospheric pressure microwave plasma in the future. The discharge characteristics were investigated by observing the discharge patterns of the plasma, and the rotational temperature of the C2 molecule in the exciting region of CO2 discharge was diagnosed by means of optical emission spectroscopy; thereby, its variations with respect to different positions, microwave power and gas flow rates were obtained. The results indicate that the discharge patterns of the atmospheric pressure microwave plasma exhibit abright central discharge region and afterglow region surrounding the central discharge region, and a clear boundary between these two regions can be observed. The length of the central discharge region increases linearly with the increase of microwave power and is weakly affected by an increase inthe gas flow rate. The diagnostic results of optical emission spectroscopy show that during the discharge process, in addition to the continuous chemical fluorescence spectrum, strong Swan bands of C2 molecules exist in the central discharge region. The plasma temperature in the central discharge region calculated based on the optical emission spectroscopy is approximately 6 000 K, and it almost no changes with varieties of power and gas flow rate and varies slightly (±100 K) at different locations of the central discharge region.