The invention discloses a simple ultrasensitive colorimetric detection method for bivalent copper ions; by using low-concentration o-phenylenediamine in the method, it is possible to effectively mediate agglutination of gold nanoparticles with surface negative charge such that reaction solution undergoes color change reaction from red to blue; a small amount of bivalent copper ions can efficiently and specifically oxidize o-phenylenediamine into 2,3-diaminophenazine, thereby inhibiting the color change reaction of the above solution. The degree of color change of the reaction solution due to agglutination of the gold nanoparticles is in reverse proportion to Cu2+ concentration in a sample. Therefore, simple ultrasensitive qualitative or semi-quantitative detection of Cu2+ may be achieved just by observing color changes of the reaction solution through naked eyes; accurate quantitative detection of target metal ion analyte may further be achieved by the aid of an ultraviolet and visible light spectrophotometer; the method of the invention is directly popularizable and applicable to the simple, economical, quick, sensitive and specific analytic detection of Cu2+ analyte in various types of samples in the fields, such as medical diagnosis, environmental supervision, and food safety.