The effect of Cu2+ on the catalytic degradation of Acid chrome dark blue (ACDB) in UV-TiO2 process and Fenton process was studied. In these two processes, Cu2+ markedly inhibited the degradation of ACDB. These inhibition effects had an obvious relationship with the concentration of Cu2+. The experimental results indicated that in pH 5.0 buffer solution, ACDB reacted with Cu2+ with the formation of new complex Cu(ACDB)2. This interaction was favorable to protect some groups in ACDB from the oxidation of reactive oxygen species generated in UV-TiO2 process and Fenton process, and consequently had suppressing effects on degradation of ACDB.