Graphitic carbon nitride supported silver nanoparticles (AgNPs/g-CN) with 1%, 3%, and 5% AgNPs were successfully synthesized by an "" method with ultrasound of a mixture of AgNP solution and g-CN. The AgNP solution was prepared by chemical reduction with trisodium citrate, and g-CN was synthesized from the urea precursor. The supported nanoparticles were characterized by X-ray diffraction spectroscopy (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), nitrogen adsorption-desorption (BET), Fourier transformation infrared (FTIR) and Raman spectroscopy, ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis DRS), photoluminescence spectroscopy (PL), electron paramagnetic resonance (EPR) and electrochemical impedance spectroscopy (EIS) Nyquist plots. The visible light-driven photocurrent measurement was performed by three on-off cycles of intermittent irradiation. The analyses show that AgNPs were evenly dispersed on g-CN, and have sizes ranging from 40 to 50 nm. The optical properties of the AgNPs/g-CN material were significantly enhanced due to the plasmonic effect of AgNPs. The photocatalytic activity of catalysts was evaluated by 2,4-D degradation under visible light irradiation ( > 420 nm). In the reaction conditions: pH 2.2; (2,4-D) 40 ppm; a m/v ratio of 0.5 g L, AgNPs/g-CN materials exhibit superior photocatalytic activity compared to the pristine g-CN. The studies on the influence of free radicals and photogenerated holes, h, show that ˙OH, O˙, and h play decisive roles in the photocatalytic activity of AgNPs/g-CN. The TOC result indicates the minimal toxicity of the by-products formed during the 2,4-D degradation. In addition, the AgNPs/g-CN catalytic activity under direct sunlight irradiation was similar to that under artificial UV irradiation. Based on these results, a possible mechanism is proposed to explain the enhanced photocatalytic activity and stability of AgNPs/g-CN. Theoretical calculations on the interaction between 2,4-D and g-CN, Ag/g-CN was also performed. The calculated results show that the adsorption of 2,4-D on Ag-modified g-CN is significantly more effective compared to pristine g-CN.