The electron density of dielectric barrier discharge in micro-discharge at atmospheric pressure argon is measured by emission spectral method. Considering various broadening mechanisms of plasma, the unsymmetrical convolution procedure is compiled to fit the experimental spectral line profile, and the Stark broadening profile can be obtained by deconvolution procedure to measure the electron density of plasma. The results show that the electron density in three filamentary micro-discharge at atmospheric pressure argon is 4.06×1021 m-3 if the electron temperature is 10000 K. The diagnosis result is in good agreement with the simulation. The method for measurement of electron density can be used not only in dielectric barrier discharge at atmospheric pressure, but also in other types of non-hydrogen plasma at atmospheric pressure.