Abstract: Organic/polymer photodetectors have attracted great attention due to their outstanding advantages, including controllable spectral, adjustable bandgap, simple processing technology, flexibility, and large area fabrication.Herein, three oligothiophene moieties 3,3′-dimethoxy-substituted bithiophene（OT）,3,4-dimethoxy-thiophene（O）,and 3,6-dimethoxythieno3,2-bthiophene（OTT） were selected respectively as electron-rich units, and pyrrole3,4-cpyrrole-1,4-dione（DPP） was used as electron-deficient unit to prepare three D-A type narrow bandgap conjugated polymers（CPs） POT-DPP,PO-DPP,and POTT-DPP.The systematic study was conducted on the thermal stability, optical absorption, aggregation, energy levels of polymers as well as the photovoltaic performance, charge transfer, recombination, and specific detectivity（D^*） of their photovoltaic photodetectors.It was found that the photodetectors based on POTT-DPP which bearing the larger conjugated planes paired with PC71BM achieved the best photodetection performance with the highest D^* of 1.87×10¹¹ Jones at 400 nm and 1.67×10¹¹ Jones at 850 nm, respectively.These results mainly contributed by the higher and balanced hole/electron mobility, well suppressed bimolecular recombination, trap assisted recombination and desired surficial morphology of photoactive layer.This work demonstrated that enlarging the conjugated area of the electron rich units of D-A conjugated polymers can effectively adjust the bandgap, promote the carrier migration, and suppress the recombination processes in their photodetectors, thereby promoting the photo detectivity of the corresponding devices.