The modifier effect on the extraction of mercury ion with sodium diethyldithiocarbamate dissolved in supercritical carbon dioxide was investigated. The extraction of the mercury ion spiked on filter paper was experimentally studied in a supercritical carbon dioxide extraction experimental installation. Several organic solvents were used as co-solvents to modify supercritical carbon dioxide. Water and HNO3 aqueous solutions were used to modify the solid matrix to be extracted. The enhancement of the exaction efficiency of the mercury ion depends on the polarity and amount of co-solvent. The extraction efficiency was also enhanced by modifying the solid matrix with water or HNO3 aqueous solution, and we found that the extraction depended on the concentration of HNO3 in the modifier for the solid.