The effects of urea on the binding of Sulfonlyurea herbicides to catalase in aqueous was studied using fluorescence spectroscopy. It was shown that herbicides has a strong ability to quench the catalase flurescence mainly through a static quenching procedure. The binding constant K and the number of binding site n were calculated according to the flurescence quenching results. For clorsufuron, K=8.69×105 L·mol-1 and n=1.16; for metsufuron methyl, K=1.01×106 L·mol-1 and n=1.20; for bensufuron methyl, K=3.52×103 L·mol-1,n=0.77. When urea used, For clorsufuron, K=2.82×102 L·mol-1,n=0.49; for metsufuron methyl, K=3.60×105 L·mol-1,n=1.14; for bensufuron methyl, K=3.46×103 L·mol-1,n=0.70. It is clear that the binding of metsufuron methyl with catalase is stronger than that of clorsufuron and the binding of clorsufuron stronger than that of bensufuron methyl ; When urea used, the binding of metsufuron methyl and clorsufuron with catalase is weaker than that when urea not used, but for bensufuron methyl the binding is same as that when urea not used.