The present invention relates to a benzodithiophene based copolymer containing pyridino[2,1,3]thiadiazole units and a preparing method and applications thereof. The polymer has a structural formula (I), wherein R1 and R2 are respectively selected from H or alkyl groups of C1 to C16; R3 and R4 are respectively selected from H, alkyl groups of C1 to C16, alkoxy groups of C1 to C16, or thiophene groups substituted by alkyl groups of C1 to C16; X is N and Y is CH, or X is CH and Y is N; and n is a natural number of 7 to 80. Applications of the benzodithiophene based copolymer containing pyridino[2,1,3]thiadiazole 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.