The invention discloses a dithiophenylacrylonitrile-based non-isotactic polymer, and a preparation method and an application thereof. The structural formula of the dithiophenylacrylonitrile-based non-isotactic polymer is represented by formula I. In the formula I, R is any one selected from C5-C80 (preferably C5-C50) linear or branched alkyl groups, and n is the polymerization degree and is 5-200.The invention also provides the preparation method of the polymer represented by formula I. The preparation method has the advantages of simple and easy synthesis route, few synthesis steps, and suitableness for large-scale synthesis. The mobility and the on/off ratio of a field effect transistor obtained by using the dithiophenylacrylonitrile-based non-isotactic polymer as an organic semiconductor layer are, the hole mobility is 1.48 cm<2>V<-1>s<-1>, and the on/off ratio is 10<3>; and the electron mobility is 1.27 cm<2>V<-1>s<-1>, and the on/off ratio is 10<3>. The polymer has a good application prospect in organic field effect transistor devices.