The　fundamental　principles　governing　photovoltaic　properties　of　nonfullerene　acceptors　(NFAs)　are　essential　for　developing　high-performance　polymer　solar　cells.　In　this　work,　using　three　heteroheptacene-based　acceptors　(Mseries)　as　model　compounds,　we　systematically　study　the　implication　of　heteroatoms　in　the　heteroheptacene　core　on　the　molecular　packing,　morphology,　optoelectronic,　photophysical,　and　photovoltaic　properties　of　the　NFAs.　It　is　found　that　replacing　the　oxygen　atoms　at　the　inner　positions　of　the　heteroheptacene　core　with　sulfur　atoms　leads　to　the　molecular　packing　mode　change　from　a　linear　two-dimensional　(2D)　brickwork　structure　to　a　threedimensional　(3D)　reticulated　motif,　which　facilitates　the　formation　of　a　desirable　active　layer　with　nanostructured　phase　separation　morphology　and　improved　charge　transport.　Meanwhile,　the　down-shifted　highest　occupied　molecular　orbital　energy　level　of　M36　induced　by　the　sulfur　substitution,　matches　better　with　that　of　polymer　donor　PM6　thereby　leading　to　more　efficient　exciton　dissociation　and　charge　transfer.　As　a　result,　the　best-performing　photovoltaic　device　based　on　M36　affords　an　outstanding　efficiency　of　17%,　which　is　among　the　highest　values　reported　for　all　the　ADA-type　NFAs.　Our　work　reveals　the　important　role　of　the　heteroatoms　in　the　heteroheptacene　core　in　constructing　the　3D　network　of　ADA-type　NFAs,　and　the　structure-property　relationships　herein　shall　provide　an　important　guidance　for　designing　high-performance　NFAs.