The static first hyperpolarizabilities (β vec ) of ligand L (N,N′-bis(4-methoxyphenyl)ethylenediimine) and its Pt(II) chelated complexes have been calculated by density functional theory (DFT) method. The results show that the β vec values of Pt(II) diimine complexes range from ∼4.2 to ∼198.8 times larger than that of ligand L, and the β vec values of Pt(II) complexes with coordinated atom O are larger than those of the corresponding complexes with S. Notably, the bimetallic complex 3a possesses the largest β vec value in the studied systems, ∼1498.86 × 10 −30 esu, which is ∼46.6, ∼30.5, and ∼3.6 times as large as those of complexes 1a, 2a and 3b, respectively. In addition, the dynamic first hyperpolarizabilities (β vec (ω)) of complex 1a are also calculated at the input photon energy from 0.1 to 1.8 eV. Our present work would be beneficial for further theoretical and experimental studies on large second-order nonlinear optical (NLO) responses of metal complexes.