Microwave and optical measurements are the best diagnostic methods for electron-beam-generated air plasma. According to the microwave characteristic of plasma, the attenuation in energy and phase shift can be adopted to diagnose electron density. The experimental results of microwave and optical measurements are coincident. From the experiment results, humidity effect on air plasma is analyzed, and it is found that the electron density of humid air plasma is obviously reduced compared to that of the dry air plasma under the same experiment conditions. The numerical calculations using the chemical processes model are completed for air plasma, and the numerical result is in a good agreement with the experimental result.