The invention relates to a saline-alkali soil improving agent and its preparation method. The preparation method comprises the following steps: mixing Rhodopseudomonas palustris, Pichia membranefaciens Hansen bacterium, lactobacillus casei, Staestomyces microflavus and seaweed slurry, organically combining a plurality of beneficial microorganisms and seaweed through multi-time fermentation, culturing in a compound mode to obtain a novel microorganism agent-seaweed bio-fungus. adding a proper amount of humic acid, organic matter, active zeolite powder and molasses, fermenting and drying the fermented product. The saline-alkali soil improving agent uses a plurality of microorganisms and employs strong effects of a unique ionic adsorbent, an organic neutralization balancing agent, an ion-exchange agent and a beneficial bacteria conversion starter to achieve the purposes of improving the soil environment, regulating acid-base balance, reducing the salt content, effectively raising the soil fertility and permeability, loosening the soil, eliminating the soil hardening, degrading the harmful substance and promoting the accumulation and conversion of the organic substance.