In recent years, polar magnets M2Mo3O8(M:3d transition metal) have emerged as a research focus in condensed matter physics and materials science due to their unique crystal structures, multiple continuous magnetoelectric-coupled state transitions, and potential applications. Notably, Co2Mo3O8 exhibits a significant second-order nonlinear magnetoelectric coupling effect in its ground state, corresponding to a unique microscopic magnetoelectric coupling mechanism and practical value. In this work, based on a molecular field phenomenological model, we construct two distinct antiferromagnetic sublattices for the Co2Mo3O8 system and present the temperature-dependent variations of its spontaneous magnetic moment, spin-induced ferroelectric polarization, first-order linear magnetoelectric coupling coefficient, and second-order nonlinear magnetoelectric coupling coefficient. Particularly, the parameters t=-1 and o=-0.96 indicate distinct exchange energies between the magnetic sublattices associated with tetrahedra (Cot) and octahedra (Coo). The Co2+ ions in these two sublattices, which were characterized by different molecular field coefficients, synergistically mediate a spin-induced spontaneous polarization of PS~0.12 μC/cm2 through the exchange striction mechanism and p-d hybridization mechanism in Co2Mo3O8. In addition, the significant second-order magnetoelectric coupling effect with a coefficient peaking at 70x10-19 s/A near the TN in Co2Mo3O8, where this coefficient is significantly larger than those of isostructural Fe2Mo3O8 (1.81x10-28 s/A) and Mn2Mo3O8, implies that this enhancement primarily arises from the weaker inter-sublattice antiferromagnetic exchange between its two sublattices, leading to a stabilizes metastable spin configuration. This also indicates that the Co2Mo3O8 system possesses stronger irreversibility stability and exhibits a pronounced magnetoelectric diode effect, providing a solid theoretical and computational foundation for the development of magnetoelectric diodes.<
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