The invention provides a preparation method of a compound organic fertilizer for a saline and alkaline land, relates to the field of fertilizers, and solves the problems that the water retention capacity is poor, the action time is short, and and long-acting and long-lasting treatment cannot be realized in the conventional organic fertilizer for a saline and alkaline land. The organic fertilizer includes 20-25 parts of excrements of people and livestocks, 35-40 parts of plant straws, 6-8 parts of bentonite, 5-8 parts of potassium fulvic acid, 8-10 parts of humic acid, 7-10 parts of porous light-weight volcanic rock, 10-15 parts of pharmacy leftovers from a pharmaceutical factory, 9-12 parts of a compound water-retaining agent, 3-5 parts of a sandy soil adhesive, 1-2 parts of bacillus subtilis, 1-2 parts of arbuscular mycorrhizal, 1-2 parts of trichoderma harzianum, 1-2 parts of salt-tolerance rhodopseudomonas capsulata, 1-2 parts of jelly bacillus, 1-2 parts of phosphate solubilizing bacteria, 1-2 parts of potassium bacteria and 1-2 parts of saccharomycetes. According to the invention, the soil structure of the saline and alkaline land is effectively improved, the fertility is improved, the saline and alkaline is reduced, the soil water retention capacity is improved, accumulation of salt is prevented, and the action is long-lasting.