In order to achieve the full response of three primary colors(red, green, blue) of organic photodetectors and the improvement of devices performances, the spectral broadening of organic active layer based on tri-phase bulk heterojunction formed by Poly(3-hexylthiophene) (P3HT)∶［6,6］-Phenyl-C61-butyric acid methyl ester (PC61BM) doped with narrow band material poly-{［4,8-bis［(2-ethylhexyl)oxy］-benzo［1,2-b: 4,5-b′］dithiophene-2,6-diyl］-alt-［3-fluore-2-(octyloxy)cabonyl-thieno［3,4-b］thiophene-4,6-diyl］} (PBDT-TT-F) was investigated. Additionally, the dark current was reduced due to the increase of electron injection barrier by the replacement of anode buffer layer so as to improve the devices performances. The influence of PBDT-TT-F doping concentration and buffer layer thicknesses on devices optoelectronic performances was discussed systemly. On this basis, the photodetectors with 400~750 nm wide spectra was obtained. At －1 V bias, the linear dynamic range and the detectivity reached 81, 80, 81 dB and 2.7×1012, 2.0×1012, 2.6×1012 Jones respectively under illumination of three primary colors. The results show that the absorption spectra of the active layer can be broadened by adding a small amount of spectral broadening material under the premise of maintaining the morphology and electrical properties of the original binary bulk heterojunction. Utilizing Molybdenum trioxide (MoO3) with optimized thickness to replace original Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)dry re-dispersiblepellets］ (PEDOT∶PSS) anode buffer layer so as to further improve the performance. Hence, the linear dynamic range and the detectivity of the devices were enhanced obviously. The present study paves the way for high detectivity and broadband organic photodetectors based on tri-phase bulk heterojunction.