Conjugated polymers (CPs) contain one pi-conjugated backbone and functional groups that could be ionized in high dielectric media. These materials combine the semiconducting and photon harvesting properties of electronically delocalized polymers with the charge-mediated behavior of polyelectrolytes. CPs can be used as highly responsive optical sensors for chemical and biological targets. The density functional theory (DFT) and the time-dependent density functional theory (TDDFT) approach were employed to simulate the absorption and emission spectra of poly[9,9'-bis(6''-N,N,N-trimethylammonium)hexyl]fluorene-co-alt-4,7-(2,1,3-benzothiadiazole) dibromide] (PFBT) in the present study. The influences on the spectra of the monomer unit F(BT)F due to stacking with the fluorene (F) and 2,1,3-benzothiadiazole (BT) units have been explored. The results suggest that stacking lowers the excitation and emission energy, facilitating detections of the polymers.