The mechanism of intramolecular singlet fission in donor-acceptor-type copolymers, especially the role of the dark 2Ag state, is not so clear. In this Letter, the electronic structure of the benzodithiophene (B)-thiophene-1,1-dioxide (TDO) copolymer is calculated by density matrix renormalization group theory with the Pariser-Parr-Pople model. We find that the dark 2Ag state is the lowest singlet excited state and is nearly degenerate with the 1Bu state. So, a fast internal conversion from 1Bu to 2Ag state is highly possible. The 2Ag state has a strong triplet pair character, localized on two neighboring acceptor units, which indicates that it is an intermediate state for the intramolecular singlet fission process. With the increase of the donor-acceptor push-pull strength in our model, this triplet pair character of the 2Ag state becomes more prominent, and meanwhile the binding energy of this coupled triplet pair state decreases, which favors the separation into two uncoupled triplet states. We propose a model in which the competition between the singlet fission process and the nonradiative decay process from the 2Ag state would determine the final quantum yield.