Molecular construction with π-conjugated groups is a favorable strategy to discover superior nonlinear optical (NLO) materials. Herein, a novel vanadium-carbonate compound K3[V(O2)2O]CO3 (KVCO), which combines two different π-orbital groups, delocalized (CO3)2- π-orbital groups and localized (O2)2- π-orbital groups, was synthesized. Benefiting from the well-arranged [V(O2)2OCO3] unit and dual-synergistic π-d-π interaction, KVCO has the largest second harmonic generation (SHG) effect (about 20 times that of KDP) among the current carbonate NLO materials. Moreover, first-principles calculation elucidated the coupling enhancement mechanism between (CO3)2-, (O2)2- and VOn groups.