A new electrochemical sensor has been constructed for ultra-sensitive detection of lead ions (Pb2+) by square wave anodic stripping voltammetry (SWASV), based on the copper sulfide/graphitic carbon nitride nanocomposite modified glassy carbon electrode (CuS/g-C3N4/GCE). First, spherical CuS nanoparticles with good electrical conductivity were anchored on layered g-C3N4 with high coordination activity, affording an excellent electrode modifier CuS/g-C3N4 nanocomposite. Then, the performance of the CuS/g-C3N4/GCE and its electrochemical response to Pb2+ were thoroughly studied, and the sensing mechanism was investigated. On the one hand, the CuS/g-C3N4 nanocomposite has greatly improved the electron transportation and electrode performance through functional complementarity - CuS endows g-C3N4 with a good electrical conductivity and a large active specific surface area, while g-C3N4 endows CuS with high dispersibility and strong adsorption. On the other hand, the CuS/g-C3N4 modifier has effectively promoted the deposition of trace Pb2+ from the solution onto the electrode surface by means of synergistic enrichment (crucial for amplification of detection signals) - g-C3N4 can coordinate with Pb2+ by its large number of conjugated triazine heterocyclic rings in its molecular framework, while CuS can adsorb Pb2+ due to its inherent size effect of nanomaterials. The proposed sensor can efficiently detect Pb2+ in the concentration range of 0.050-5.000 μM with a limit of detection (LOD) as low as 4.00 nM, and can be well applied for the detection of trace Pb2+ in actual tea samples.