High quality interference pattern was obtained with a modified Mach-Zehnder interferomenter. An FFT analysis is applied to extracting the phase of the reconstructed interferograms, and then the inverse Abel transformation is used to calculate the three-dimensional electron density distribution. The electron density and the plasma expanding velocity at various delay times were obtained. The results show that the formation of the plasma channel in the early stage of the laser induces air plasma, and the rapid decrease of the plasma expanding velocity accelerates the collapse of the plasma channel. At about 48 nanosecond time, the eccentricity of the plasma shape reaches the maximum value and then evolves to a circular shape.