The invention provides a high-breakdown-strength polyimide thin film which comprises an oleic acid/acrylonitrile-containing random copolymer and polyimide. A preparation method of the oleic acid/acrylonitrile-containing random copolymer comprises the following steps: (1) stirring oleic acid, acrylonitrile, an emulsifier, an initiator and water in a container A to obtain pre-emulsion; (2) mixing 10-30wt% of the pre-emulsion and water, and carrying out reaction in a container B to obtain seed emulsion; (3) adding the rest of the pre-emulsion in the container A into the container B; (4) after the reaction is completed, carrying out cooling and filtering to obtain the high-breakdown-strength polyimide thin film.