Supercritical carbon dioxide (SC CO2) has shown great potential in fabrication of two-dimensional (2D) amorphous nanomaterials with excellent electric and optical properties, while the amorphization mechanism led by SC CO2 is still unclear. In this work, by investigating the amorphization kinetics of MoO3-x nanomaterials in SC CO2, we find two amorphization mechanisms dependent on the SC CO2 pressure. At lower pressure, forming oxygen vacancies is the dominant effect, while at higher pressure, atomic rearrangement is the controlling factor. Furthermore, we demonstrate that amorphization directly affects the optical performance of MoO3-x nanosheets because of the change in coordination, which further indicates the atomic rearrangement during the amorphization process. Therefore, this work reveals the amorphization mechanism led by SC CO2 and builds a link between amorphization and optical performance; it also provides new inspiration for fabrication of amorphous nanomaterials with tunable optical and photocatalytic performance.