This paper reports a novel colorimetric sensor for pymetrozine based on p-aminobenzenesulfonic acid functionalized silver nanoparticles (p-ABSA-modified Ag NPs), which were characterized by IR spectroscopy, ultraviolet–visible spectroscopy (UV–vis), and transmission electron microscopy (TEM). The newly synthesized p-ABSA-modified Ag NPs are yellow in color due to the intense surface plasmon absorption band centered at 400nm. In the presence of pymetrozine, the yellow p-ABSA-modified Ag NPs solution turns to dark green, with a dramatic surface plasmon absorption band centered at 660nm. Moreover, high selectivity for pymetrozine was approved by the comparative experiments with an absorption ratio of A660/A400 more than 0.7. This highly sensitive sensor allows a direct and rapid quantitative assay of pymetrozine with a colorimetric limited detection concentration of 0.01mg/L.