Phenacetin is often illegally adulterated into herbal teas for enhanced analgesic and anti-inflammatory properties, posing great threats to consumers' health. However, few rapid tests are available to monitor the banned phenacetin. Though one phenacetin immunoassay has been reported already, its unmet selectivity limits the wider application in rapid screening. Herein, a highly specific and sensitive phenacetin antibody was induced by long spacer arms-based immunogens coupling with low-electronegativity coating antigen. Furthermore, a precise indirect competitive enzyme-linked immunosorbent assay (ic-ELISA) and a field practical colloidal gold immunochromatographic assay (AuNPs-LFIA) were developed for phenacetin detection in Guangdong herbal teas. This work exhibits low detection limits (8.80 ng/mL for ic-ELISA, 2.19 ng/mL for AuNPs-LFIA) and reasonable recoveries (102.8%-111.1% and 91.68%-111.91%). It is noteworthy that both of the developed ic-ELISA and AuNPs-LFIA have high accuracy and practicability in blind herbal tea analysis, which was validated by the authorized liquid chromatography-mass spectrometry/mass spectrometry method. The proposed ic-ELISA and AuNPs-LFIA could be promising approaches in monitoring Guangdong herbal teas with potential illegally adulterated phenacetin. PRACTICAL APPLICATION: Rapid and accurate monitoring of illicitly added drugs of phenacetin in herbal teas is necessary and important for consumers and regulatory agencies.