Narrow-band response photodetectors are widely used in situations where specific light wavelengths need to be detected, such as communication, imaging, surveillance, etc. These detectors require not only the narrowest spectral response window, but also the highest external quantum efficiency (EQE) within the response window. Generally, the semiconductor photoactive layer in the photodetector will absorb the photons with energy greater than the band gap, and the spectral response window is usually wide, so it needs to combine with the filter of specific wavelength to achieve narrow band response. However, the introduction of filters, in addition to the increased cost, will inevitably lead to additional light reflection losses, which will significantly reduce EQE. When the film thickness of the heterojunction is up to μm, the narrow-band response near the band edge can be achieved without a filter. However, the problem brought by thick film is that the photocurrent also decreases, resulting in lower EQE (<30%) of the device. Although higher reverse bias increases the photocurrent and EQE, it also increases the collection probability of short-wave photogenerated carriers, thus damaging the narrow-band response. By greatly reducing the proportion of organic receptors and donors in the micron-size photoactive layer, the narrow-band response window of ca. 650 nm was achieved, and the EQE was up to ca. 53500% under –60 V reverse bias. This kind of organic photodetector achieving the photomultiplication mainly attributes to the trapped charges at the photoactive layer/electrode interface which will capture the photogenerated electrons (holes), and when the photogenerated electrons (holes) are collected, the electric neutral balance in the photoactive layer is broken. The trapped photogenerated electrons (holes) at the interface will cause the change of interface potential energy, which induces the interface band bending of the photoactive layer, and promotes the metal electrode to inject holes into the photoactive layer to maintain the neutral balance. Because the injection current is much higher than the photoelectric current, the superposition of the two can significantly increase EQE and even produce gain. However, the idea of obtaining higher EQE by further increasing the concentration of interfacial trapped electrons is difficult to be realized by increasing the proportion of receptors, which not only reduces EQE, but also increases the full width at half maximum (FWHM) of the response window and destroys the narrow-band response. In this work, the structure of ITO/poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT)/poly(3-hexylthiophene) (P3HT)/[6,6]-phenyl-C61-isomethyl butyrate (PCBM)/aluminum (Al) has been prepared. By optimizing the thermal anneal-ing time and PCBM concentration, the EQE at –60 V was successfully improved from 52946.8% to 68470.9%, which is the highest gain of narrow-band response organic photodetector so far, and the FWHM of ca. 28 nm remains unchanged. We used Nano-IR to confirm that EQE enhancement was attributed to thermal annealing induced diffusion of PCBM into P3HT, thus optimizing the electron trap concentration distribution at the interface between the active layer and the metal top electrode and enhancing carrier injection.