The invention relates to the field of water and soil conservation in agriculture and forestry, and particularly relates to a humic acid water-retaining agent for vegetables and a preparation method thereof. The humic acid water-retaining agent is quick to prepare, low in cost and high in potassium content. The water-retaining agent disclosed by the invention is applied to vegetable crops, so that the moisture utilization rate can be improved, the soil fertility is ensured, growth and development of vegetables are stimulated, the vegetable nutrition function is improved, the stress resistance of the crop is increased, and both production and income are increased. The humic acid water-retaining agent for the vegetables comprises the following raw materials in parts by weight: 1-5 parts of decomposed coal, 40-70 parts of 0.1mol/L of sodium hydroxide solution, 2-8 parts of sodium hydroxide of which the purity is 98%, 0.5-4 parts of potassium hydroxide of which the purity is 98%, 5-20 parts of crylic acid, 2-8 parts of acrylamide, 0.01-0.05 parts of N,N'-methylene bisacrylamide, 0.4-2 parts of potassium peroxodisulfate and 0.5-5 parts of self-initiation heating agent. The humic acid water-retaining agent is prepared by adopting a specific method.