The invention discloses a cloud and rain microphysical parameter inversion method based on a space-borne three-frequency millimeter wave radar. The method comprises the following steps of firstly, calculating and analyzing cloud and rain particle temporal and spatial distribution characteristics and summarizing, using a generalized Gamma distribution function to characterize cloud raindrop particle distribution and carrying out parameterization; then, based on an idea of a Bayesian estimation theory, establishing an inversion model, presetting prior distribution of the parameters, inputting space-borne W, Ka and Ku three-frequency millimeter wave radar reflectivity factors into a physical model, inputting space-time matching ground-based millimeter-wave radar data as an adjustment factor,carrying out posterior, after iterative calculation, continuously correcting prior probability distribution to minimize the cost function and outputting an optimal inversion result; and finally, carrying out linear processing on an inversion result in each distance library to obtain the inversion result of an entire profile. In the invention, a space-time resolution is high, cost is low, the inversion result is fine and actual distribution of cloud and rain particles can be simultaneously inverted.