Exploring chromophores which determine band gap sizes and nonlinear optical (NLO) activity is of importance for material design. In this study, the electronic structures and optical properties of a class of the semiorganic NLO materials ASr[C₄H₂O₆B(OH)₂]·4H₂O (A = K and Rb) and simulated virtual compounds ASr[C₄O₆B(OH)₂]·4H₂O (A = K and Rb) with CC bonding after removing H atoms have been investigated to clarify the role of microscopic units. It was found that introducing π-conjugated microscopic units could increase the SHG effect but decrease the band gap. Whether the band gap red shift or the electronic transfer induced by π-conjugated microscopic units should be responsible for the change of the SHG effect is explored by combining the electronic structure, SHG density and molecular orbital analysis.