The invention discloses a humic acid bio-organic fertilizer with water retention function and a preparation method of the humic acid bio-organic fertilizer. The humic acid bio-organic fertilizer comprises an organic material, a decomposition agent, a functional microbial agent, a water and fertilizer retention agent and a filler; the organic material comprises 450-550 parts of cassava residue, 200-300 parts of sugar mud, 80 parts of humic acid and 30 parts of amino acid; the decomposition agent comprises bacillus methylotrophicus and bacillus amyloliquefaciens; the functional microbial agent is prepared from bacillus methylotrophicus, 0.5 to 5 parts of a water-retaining fertilizer-retaining water-absorbent resin, 0.5 to 5 parts of a nonionic polyacrylamide, 0.5 to 5 parts of carboxymethylcellulose and 10 to 25 parts of expanded starch; the filler is prepared from the following components in parts by weight: 10 to 25 parts of kaolin, 10 to 20 parts of montmorillonite and 10 to 20 partsof bentonite. Biostimulants humic acid and amino acid are effectively applied to production of the water-retaining bio-organic fertilizer to prepare a functional bio-organic fertilizer product, so that the physical and chemical properties of soil are improved, water and fertilizer are retained, healthy and strong growth of crops is stimulated, the quality and yield of the crops are improved, andsustainable development of agriculture is promoted.