The invention provides a preparation and purpose of am isoindigo derivative and thiophene copolymer using thiophene as a kernel. The structure of the copolymer is as shown in a formula I, (as shown in the description), wherein R1 is C8-C20 alkyl chains, and n is greater than 1 and smaller than or equal to 100. A synthetic method provided by the invention is simple to operate, and the cost of raw materials is low. The copolymer prepared by the synthetic method disclosed by the invention can be used as an organic semiconductor layer to be applied to polymer solar battery tests. Materials can be processed in a solution, ultraviolet spectrum absorption performance is excellent, film photovoltaic device tests indicate that polymeric materials can be applied to film photovoltaic device active layers, and in device tests of inversion structures, the photoelectric transformation efficiency is 3.62%.