The invention provides a preparation method of a lactose-free fermented dairy product. The method comprises the following steps: blending, homogenizing, sterilizing, carrying out enzyme-hydrolyzing, fermenting, filling and refrigerating all the raw materials in sequence to obtain the lactose-free fermented dairy product, wherein the enzymatic hydrolysis and fermentation steps comprise the following processes: adding lactase at the temperature of 35-40 DEG C, hydrolyzing till the degree of hydrolysis is above 80 percent, and then adding stains; or adding lactase at the temperature of 41-45 DEG C, hydrolyzing till the degree of hydrolysis is above 95 percent, and then adding stains; fermenting till the pH value of feed liquid is 5.5, starting to inactivate the lactose and ending the fermentation when the pH value of the feed liquid is 4.3-4.5; or adding lactose and medium temperature strains at the temperature of 30-35 DEG C, fermenting till the pH value of feed liquid is 5.5, then rising the temperature to 40-45 DEG C and adding thermophilic stains, and ending the fermentation when the pH value of the feed liquid is 4.3-4.5. The preparation method has the benefit that through addition of the lactose and improvement of a fermented milk preparation process, the lactose-free fermented dairy product can be effectively prepared and obtained.