Ideal morphological features are of particular importance to produce high performance all-polymer solar cells (all-PSCs), in which active blends generally involve unfavorable phase separation due to complicated intermixing. Developing a suitable processing solvent and additive is an effective and versatile approach to manipulate the morphology of the blends. This study demonstrates the synergistic effects of the processing solvent and additive on the photovoltaic performances of all-PSCs composed of a conjugated copolymer J71 donor and a typical N2200 acceptor. A low boiling point chloroform (CF) solvent combined with 1% 1,8-diiodoctane (DIO) additive was identified as the optimal processing condition to treat the J71:N2200 blends. Consequently, the all-PSCs prepared from CF + 1% DIO processing achieved an outstanding efficiency of 9.34% with an ultrahigh fill factor of 77.86%, which is among the top values for the current all-PSC systems. Owing to the low JSC, just a moderate efficiency of 7.28% was achieved for the device prepared from chlorobenzene (CB) + 1% DIO processing despite its high FF. The electron microscopy tests revealed that the CF solvent was superior to the CB solvent to obtain uniform morphologies and the addition of the DIO additive could further lead to more favorable phase separation and domain size. Moreover, the results of charge generation, transport, and recombination analysis correlate well with the remarkable photovoltaic properties. Our results highlight the critical significance of selecting the appropriate processing solvent and additive to produce high performance all-PSCs.