Stretchable organic solar cells (OSCs) have great potential as power sources for the next-generation wearable electronics. Although blending rigid photovoltaic components with soft insulating materials can easily endow the mechanical ductility of active layers, the photovoltaic efficiencies usually drops in the resulting OSCs. Herein, a high photovoltaic efficiency of 15.03% and a large crack-onset strain of 15.70% is simultaneously achieved based on a ternary blend consisting of polymer donor poly[(2,6-(4,8-bis(5-(2-ethylhexyl-3-fluoro)thiophen-2-yl)-benzo[1,2-b:4,5-b’]dithiophene))-alt-(5,5-(1’,3’-di-2-thienyl-5’,7’-bis(2-ethylhexyl)benzo[1’,2’-c:4,5-c’]dithiophene-4,8-dione)] (PM6), non-fullerene accepter 2,2’-((2Z,2’Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2’’,3’’:4’,5’]thieno[2’,3’:4,5]pyrrolo[3,2-g]thieno[2’,3’:4,5]thieno[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (Y6), and soft elastomer polystyrene-block-poly (ethylene-ran-butylene)-block-polystyrene (SEBS) through the control of phase separation and crystallization. By employing a high-boiling point solvent additive 1-chloronaphthalene (CN) with different solubilities for PM6 and Y6, the aggregation dynamics of PM6 and Y6 as well as the film solidification process are dramatically altered, allowing for the different molecular rearrangement and liquid–liquid phase separation evolution. Consequently, the ternary film with optimal CN content presents decreased SEBS domains and moderately improved molecular ordering of PM6 and Y6, enabling effective mechanical deformation and charge generation/transport. The revealed corrections between the film-formation process, film microstructure, and photovoltaic/mechanical characteristics in the ternary blend provide deep understanding of the morphology control toward highperformance stretchable OSCs.