Preparation method of waterborne one-component polyurethane paint with low viscosity

A preparation method of waterborne one-component polyurethane paint with low viscosity comprises the following steps: firstly, polymeric polyol and polyisocyanate react in a ratio of 1:(1.8-3.4) to generate an isocyanate-group terminated oligomer (a); secondly, low molecular polyol and polyisocyanate react in a ratio of 1:(0.3-0.5) to generate a hydroxyl terminated oligomer (b); the oligomer (a), the oligomer (b) and a polylactic acid oligomer react in a ratio of 1:(0.78-0.92):(0.02-0.95) to obtain waterborne polyurethane resin (c), wherein the molecular weight of the polylactic acid oligomer is 800-1,200 g/mol; and finally, a product is obtained through self-emulsification. According to the waterborne polyurethane paint prepared with the method, the viscosity is smaller than 60 s, the storage life is longer than 3 months, the solid content is higher than 40%, the water resistance of condensate is high, the insulativity is high, the mechanical strength is excellent, and the paint can serve as surface insulation paint for a transformer, a charger and a motor coil.