From the miscibility viewpoint, the mechanism of how the third polymer influences the morphology evolution of an active layer composed of a polymer donor and a polymer acceptor remains to be unknown. In this article, J71 was incorporated into the PBDB-T:PNDI-2T-TR(5) system to afford a ternary all-polymer solar cell showing the maximal power conversion efficiency of 9.12%. It is observed that J71 could provide complementary absorption in addition to the formation of cascade energy-level alignment with other polymers, demonstrating a Forster resonance energy transfer from J71 to PBDB-T. By the combination of dynamic mechanical analysis and differential scanning calorimetry techniques, the miscibility in the J71:PNDI-2T-TR(5) blend is better than that in the PBDB-T:PNDI-2T-TR(5) blend according to Fox equation analysis, showing Flory-Huggins interaction parameters of -1.003 and 0.006, respectively. J71 could serve as a compatibilizer to optimize the morphology of the active layer, leading to enrichment of PNDI-2T-TR(5) on the film surface due to the much lower surface energy of PNDI-2T-TR(5). The optimized vertical distribution of PNDI-2T-TR(5) with enhanced crystallization could be further demonstrated by grazing incidence wide-angle X-ray scattering and atomic force microscopy, facilitating electron transfer and collection in the normal device structure for improved photovoltaic performance.