The invention discloses a saline-alkali soil improving agent, and a preparation method thereof. The preparation method comprises following steps: fermented plant fiber and fermented animal excreta are uniformly mixed and grinded so as to obtain a mixture of 80 to 100 meshes; wood vinegar and water are added, uniform mixing is carried out, and an obtained mixed material is subjected to pelletizing and drying until water content ranges from 3 to 8%, wherein the adding amount of water accounts for 5 to 30% of the total weight amount of the mixture; an obtained product is sieved so as to obtain a granular mixture with a particle size ranging from 3 to 5mm; the granular mixture is subjected to drying and cooling, and a microorganism agent is sprayed onto the surface of the granular mixture; and an obtained semifinished product is uniformly mixed with a mixture A and a mixture B so as to obtain a finished product, wherein the mixture A is obtained via uniform mixing and grinding of ground phosphate rock, vermiculite, medical stone, sodium alginate, and polyacrylamide to 120 to 200 meshes, and the mixture B is obtained via uniform mixing and grinding of humic acid and furfural residue to 80 to 100 meshes. The saline-alkali soil improving agent is capable of reducing soil alkalinity, activating soil, reducing salinity and alkalinity effectively, improving saline-alkali soil structure, improving soil fertilizer conservation and water-holding capacity, improving saline-alkali soil physical and chemical properties, and realizing positive cycling of saline-alkali soil.