Zeolitic imidazolate framework-8 (ZIF-8), a metal-organic framework (MOF) with a non-centrosymmetric crystal structure, exhibits nonlinear optics (NLO) properties and can act as the nanoporous matrix of guest molecules. Amorphization of ZIF-8 can be achieved by pressure or high temperature. Both crystalline and amorphous states have their inherent features for optical applications. The effects of the crystalline-amorphous transition on the structural and optical properties under pressure were investigated in detail. Amorphization leads to the destruction of the ZIF-8 lattice structure, collapse of pores, and the change of spatial symmetry, which in turn alters the NLO properties of ZIF-8 and the luminescence properties of the guest Eu cations. Our results establish the structure–optical properties relationship in the amorphization process and provide new clues in designing novel MOFs optical materials.