Double-exposed holographic interferometry is an effective means in the plasma number density of electronic diagnosis, for it has such merits as non-contact measurement and non-disturbance by plasma luminescence. According to Abel transform, simulated interferograms of various plasma′s electron densities are obtained;by double-exposed holographic interferometry, interferograms of various spacecraft microwave plasma thruster′s plumes are gotten.The electron density of plume is gained by analyzing and contrasting the measured interferograms and simulated ones.