The distribution properties of Microwave Plasma Thruster′s plume are studied with the digital holographic interferometry.By numerically reconstructing the digital hologram,the phase distribution of MPT′s plume is obtained, from which the electron density is also calculated.By employing the phase multiplication method,the fringe density of the interference pattern obtained by digital hologram was multiplied to reveal the refractive index distribution of the plume field more intuitively.It is shown that the digital holographic interferometry is an effective method for diagnosing MPT plasma′s plume.