The invention relates to a sensor, in particular to an electrochemical sensor for determining the concentration of heavy metal ions in a water sample. The electrochemical sensor specifically comprises a working electrode, a reference electrode and an auxiliary electrode, and the working electrode, the reference electrode and the auxiliary electrode are respectively connected to a control potential meter through conducting wires; the working electrode consists of a polytetrafluoroethylene pipe and a carbon electrode clamped therein, the bottom end of the carbon electrode is connected with a lead wire for leading out, and a sensitive membrane is arranged at the top end of the carbon electrode; and the sensitive membrane consists of a nano-functional absorbing material, a heavy metal ion carrier and an ion exchange agent, 1-20mmol of nano-functional absorbing material, 0.5%-5% by weight of the ion exchange agent and the 0.5-10mmol of the ion carrier are added in each liter of dispersant and evenly mixed, and then 1mu L-30mu L of the mixture is taken and evenly coated on the surface of the carbon electrode. The electrochemical sensor has low cost, simple technology, high selectivity and sensitivity and good stability, and can be extensively applied in the detection of the heavy metal ions in waste water, fresh water, sea water, daily necessities and body fluid.