Cation substitution has been proven to be an effective strategy to explore novel infrared (IR) nonlinear optical (NLO) materials. In this research, a class of alkali-metal chalcogenides with different A-site cations, AInS(2) (A = Na, K, Rb, and Cs), have been successfully obtained using the first-principles calculation method by the A-site cation substitution of the excellent NLO material LiInS2. Interestingly, only NaInS2 maintains the same structure with the benchmark LiInS2. A(II)InS(2) (A(II) = K, Rb, and Cs), in spite of keeping the same space group Pna2(1), are distinct from LiInS2 due to the difference in the coordination number of cations (from 4 to 6). All compounds exhibit strong second harmonic generation (SHG) responses, and among them, CsInS2 has a comparable SHG response to that of AgGaS2. In addition, the contribution of microstructure motif to SHG responses has been determined by the method of band-resolved chi((2)) and its integral value, as well as SHG density. The results reveal that the InS4 tetrahedra play a major role in the SHG effect. Remarkably, there is a correlation between the volume, band gap, and the largest effective SHG coefficient d(32), which suggests that the variation of d(32) and band gap could be a volume effect. Therefore, volume modulation would be an effective way to tune the two crucial NLO properties of band gap and SHG coefficient in the exploration of new IR NLO materials.