The present invention relates to a benzodithiophene based copolymer containing isoindoline-1,3-diketone units and a preparing method and applications thereof. The polymer has a structural formula(I), wherein R 1 and R 2 are respectively selected from Hor alkyl groups of C 1 to C 16 ; R 3 and R 4 are respectively selected from H, alkyl groups of C 1 to C 16 , alkoxy groups of C 1 to C 16 , or thiophene groups substituted by alkyl groups of C 1 to C 16 ; R 5 is selected from alkyl groups of C 1 to C 16 ; n is a natural number from 7 to 80. Applications of the benzodithiophene based copolymer containing isoindoline-1,3-diketone units in polymer solar cells, polymer organic light-emitting, polymer organic field effect transistors, polymer organic optical storage, polymer organic nonlinear materials or polymer organic laser are also provided.