Ternary polymer solar cells (PSCs) with the third component as guest offer exceptional photovoltaic performance. Most guests in highly efficient ternary PSCs are small molecular acceptors. Utilizing polymer donor as an effective guest to improve power conversion efficiencies of PSCs remains difficult to anticipate. Herein, new asymmetrical electron-deficient thieno[3,4-d]pyridazin-1(2H)-one monomers polymerize with benzo[1,2-b:4,5-b'] dithiophene monomers, yielding two asymmetrical donor polymers, namely, PDHP-Th (with thiophene arm) and PDHP-Ph (with benzene arm). The asymmetrical thieno[3,4-d]pyridazin-1(2H)-one blocks twist the polymer backbones that induce blue-shift absorption spectra and regulate aggregations compared to that of the well-known symmetrical polymers PBDB-T and PM6. PBDB-T, PDHP-Th, and PDHP-Ph as polymer donor guests are introduced into PM6:Y6 to fabricate ternary PSC devices. The PDHP-Th polymer exhibits a similar energy level to PM6 and can mix well with the PM6:Y6 system and regulate overaggregation of the acceptor Y6, which helps to balance electron and hole mobilities in the ternary devices. All of these contributions result in a power conversion efficiency of 16.8% for the PM6:PDHP-Th:Y6 ternary PSCs. Compared to the widely reported small molecular acceptor guest approaches, our work provides a guideline and molecular design strategy for developing polymer donor guests in constructing high-performance ternary PSCs.