The invention relates to a light-weight equal-diameter micro-porous corundum complex-phase sphere and a preparation method thereof. The preparation method includes the following steps: (A) employing 90-99 wt% of Al2O3 micro-powder and 1-10 wt% of MgO micro-powder as raw materials, adding a saccharide polymer accounting for 5-15 wt% of the raw materials, and mixing the components in a planetary ball mill; (B) adding water accounting for 6-20 wt% of the raw materials, performing rotary granulation to obtain pre-fabricated spheres; (C) mixing aluminum oxide sol and [rho]-Al2O3 micro powder according to mass ratio of (0.04-0.1):1 to prepare slurry, spraying the slurry uniformly onto the surfaces of the pre-fabricated spheres, wherein a ratio of the thickness of the slurry sprayed onto the surfaces of the pre-fabricated spheres to the radials of the pre-fabricated spheres is (0.05-0.12):1; and (D) drying the spheres and calcining the spheres at 1900-2100 DEG C for 2-5 h in an ultrahigh-temperature vertical kiln to prepare the light-weight equal-diameter micro-porous corundum complex-phase sphere. The light-weight equal-diameter micro-porous corundum complex-phase sphere is low in volume density and is less in average pore diameter, is high in closed porosity, is low in heat conductivity, is good in thermal shock stability, is washing-resistant and is strong in anti-slag-erosion capability.