Fipronil (FP) is a very effective phenylpyrazole insecticide and is now widely used in agriculture. At the same time, the water and soil in the environment are polluted by FP. For the rapid detection of FP toxicants in food and the environment, we have designed an entirely novel electrochemical immunosensor that employs the combined functionalities of a cMWCNTs-AgNPs-CS-FAb-BSA layer to modify an SPCE by the freeze-drying technique. The high porosity of chitosan (CS) coupled with an excellent electron transfer enabled by the cMWCNTs and AgNPs increased the surface area for anti-fipronil (FAb) antibody immobilization and enhanced the current signal of the immunosensor. Cyclic voltammetry (CV) was applied for the quantitative determination of FP under optimized conditions (0.1 M PBS, pH 7.5, 35 °C incubation temperature, and 40 min incubation duration). The modified electrochemical immunosensor displayed excellent analytical performance, including a wide linear concentration range from 0.1 to 1000 ng mL⁻¹ with a very low limit of detection of 0.021 ng mL⁻¹ and good reproducibility (RSD = 2.58%, n = 6), stability (80.4% sensitivity after 5 days), and selectivity. Not only could the modified electrochemical immunosensor be applied in the FP residue analysis of agricultural products, but the present immobilization strategy can also potentially be applied to different biomolecules.