Size uniformed and well solution-dispersed lead sulfide(PbS) Quantum Dots(QDs) were synthesized, which were used as absorbing sources of the infrared illumination due to its absorption peak in the near-infrared spectral region. Pentacene was one of the popular organic compounds, which was easy to form films and had good electrical property. The two semiconductor materials were fabricated into a composite film as the active layer in the top-gate bottom-contact type Field-Effect Transistor(FET)-based infrared photodetectors Au(S,D)/PbS QDs/Pentacene/PMMA/ Al(G). In dark and under 980 nm illumination, the electrical and detecting parameters were measured, and the carriers transmission mechanism of the device was analyzed. A kind of PbS QDs/Pentacene composite film FET-based infrared photodetector with excellent electrical and detecting performances was obtained. Under the irradiation of a 0.1 mW/cm2 infrared laser, the responsivity of the device reached 49.4 mA/W, and the corresponding detectivity was 1.7×1011 Jones.