Abstract: Thiamethoxam (TMX), as a widely used neonicotinoid pesticide, poses potential threats to ecosystems and human health due to its environmental residues, necessitating the development of rapid and sensitive detection methods. In this study, an electrochemical sensor for highly sensitive detection of TMX was constructed for the first time using a composite of W_4/3CTₓ i-MXene (W_4/3C) with an ordered vacancy structure and graphitic carbon nitride (g-C₃N₄, gCN). W_4/3C nanosheets were prepared by HF-LiF etching of the in-plane chemically ordered i-MAX phase (W_2/3Y_1/3)₂AlC precursor, and then combined with protonated g-CN nanosheets through electrostatic self-assembly followed by hydrothermal treatment to obtain the W_4/3C/gCN composite. The structure and morphology of the materials were characterized by XRD, SEM, TEM and XPS, confirming the successful construction of W_4/3C/gCN. Electrochemical tests showed that the charge transfer resistance (R_ct) of the W_4/3C/gCN modified glassy carbon electrode (W_4/3C/gCN GCE) was only 73 Ω, significantly lower than that of electrodes modified with the individual materials. Under optimized conditions (pH=7.0), the sensor exhibited good linear relationships between the reduction peak current and TMX concentration in the ranges of 0.1~45 μmol·L⁻¹ and 45~150 μmol·L⁻¹, with a detection limit as low as 0.06 μmol·L⁻¹ and a sensitivity of 11.01 μA·L·μmol⁻¹·cm⁻². The sensor demonstrated good anti-interference ability against common ions and organic substances. After 20 consecutive measurements, the response current remained at 95.5% of its initial value, and after storage at room temperature for 90 days, it still retained 97.5% of its initial response. Spike recovery experiments in spinach, cucumber, apple and banana samples yielded recoveries ranging from 98.0% to 109.0% with relative standard deviations less than 3.91%. This study provides a new idea for the application of tungsten-based MXenes in the field of electrochemical sensing and offers a feasible approach for the rapid detection of pesticide residues.