Random ternary copolymerization is a simple but effective strategy to develop new efficient polymer donors for polymer solar cell application. Herein, a series of new copolymers, OPz1, OPz2, OPz3, and OPz4, containing varying fraction of ester-substituted thiazole (E-Tz) as the third component to the widely studied PM6 copolymer, were synthesized. Incorporation of the E-Tz unit is found to significantly improve the molecular planarity, energy levels, and light-harvesting range of the polymer donors. Impressively, subtle regulation of the E-Tz content realized a dramatic enhancement of all the device parameters with a champion power conversion efficiency of 16.28% achieved with the OPz1:Y6 device. The performance improvement was attributed to the reduced energy loss, the downshifted energy levels and optimized charge transport. Our results demonstrated that the E-Tz unit is a promising building block, and random ternary copolymerization is a powerful strategy for photovoltaic materials development.