Two acentric chalcogenide compounds, K 2 CdGe 3 S 8 and K 2 CdGe 3 Se 8 , were synthesized via conventional high-temperature solid-state reactions. The crystal structures of K 2 CdGe 3 S 8 and K 2 CdGe 3 Se 8 were accurately determined by single-crystal X-ray diffraction and crystallize in the K 2 FeGe 3 S 8 structure type. K 2 CdGe 3 S 8 is isostructural to K 2 FeGe 3 S 8 with superior nonlinear optical properties. For the second harmonic generation (SHG) response, K 2 CdGe 3 S 8 is 18× K 2 FeGe 3 S 8 for samples of particle size of 38-55 μm. The superior nonlinear optical properties of K 2 CdGe 3 S 8 over K 2 FeGe 3 S 8 are mainly contributed by the chemical characteristics of Cd compared with Fe, which are elucidated by nonlinear optical property measurements, electronic structure calculations, and density functional theory calculations. The [CdS 4 ] tetrahedra within K 2 CdGe 3 S 8 exhibit a higher degree of distortion and larger volume compared to the [FeS 4 ] tetrahedra in K 2 FeGe 3 S 8 . This study possesses a good platform to investigate how d-block elements contribute to the SHG response. The fully occupied d 10 -elements are better for SHG susceptibility than d 6 -elements in this study. K 2 CdGe 3 S 8 is a good candidate as an infrared nonlinear optical material of high SHG response (2.1× AgGaS 2 , samples of particle size of 200-250 μm), type-I phase-matching capability, high laser damage threshold (6.2× AgGaS 2 ), and good stability.