The invention relates to a preparation method of microgel acrylonitrile-butadiene rubber. The microgel acrylonitrile-butadiene rubber consists of the following components in parts by weight: 100 parts of a monomer, 2.5-6.0 parts of an emulsifier, 0.2-1.0 part of an initiator, 0.1 part of triethanolamine and 0.3-0.7 part of a molecular weight regulator, wherein the monomer consists of 50-60 parts of butadiene and 40-50 parts of acrylonitrile; the emulsifier is a composite system consisting of disproportionated potassium rosinate and potassium oleate soap; the dosage of the emulsifier is 2.5-6.0 parts; the dosage ratio of the disproportionated potassium rosinate to the potassium oleate soap is (3-8):1; the initiator is persulfate; and the molecular weight regulator is a butyl or alkyl hydrosulfide molecular weight regulator, and is added equally in three batches. Due to the adoption of the method, the acrylonitrile-butadiene rubber of which the microgel content is 1-10 percent, the Mooney viscosity MS (1+4) at the temperature of 100 DEG C is 40-65, the tensile strength is more than or equal to 27 MPa and the elongation at break is more than or equal to 400 percent can be produced.