The synthesis of a new triazene reagent 1-(4-antipyrine)-3-( 3-nitroaniline)-triazene (ANTA) and its color reaction with Cu (Ⅱ) was studied and a new spectrophotometric method for the determination of Cu (Ⅱ) was established. The results showed that in the presence of micromulsion Tween-80, ASTA could reacted with Cu ( Ⅱ) to form a red stable complex of which the molar ratio of reagent︰metal was 2︰1 in Na2B4O7-NaOH medium.. The maximum absorption peak of the complex was located at 522 nm with the apparent molar absorption of 1.5 ×105 L•mol-1•cm-1. Beers law was obeyed in the range of 0.3～5.5 μg/10 mL for Cu (Ⅱ) and the detection limit was 0.10 mg/L. Many ions did not interfere the colour reaction system due to the existence of ammonia. This method was applied to determination of Cu (Ⅱ) in water samples, and the results were in accordance with those obtained by AAS method. Recovery was in the range of 103.1～103.4 % and the RSD(n=6) was not more than 4.2%.