Disclosed in the present invention is a nonlinear optical crystal. The chemical formula of the nonlinear optical crystal is MHgGeSe4, M being selected from Ba or Sr. The nonlinear optical crystal has no symmetrical center, belongs to an orthorhombic crystal system, and has a space group Ama2. The nonlinear optical crystal is an infrared nonlinear optical crystal, and has the advantages of great nonlinear optical effect, wide light transmitting band, high hardness, good mechanical properties, breakage resistance, deliquescence resistance, easiness in processing and preserving, etc. Also disclosed in the present invention are a method for preparing the nonlinear optical crystal and application thereof.