Herein, a technique for plasma diagnosis via laser interference is described in terms of basic principles, experimental setup, and image-processing methods. Air-switch plasma is diagnosed using a Mach-Zehnder laser interferometer. The results show that the experimental device can record high-quality interferograms. Through numerical processing of these interferograms, the three-dimensional electron density distribution in plasma is reconstructed. After the air switch is ignited for 75 ns, the average electron density in plasma reaches 2×10 18 cm -3, the electron temperature is 0.11 eV, the plasma radius is 1.1 mm, and the diffusion velocity is 1.4×10 4 m/s.