Anti-idiotypic antibodies (AIds) can mimic antigen molecules and can thus offer an alternative to conventional antigens in immunoassays. In this study, citrinin (CIT) was chosen as a target analyte, and an anti-idiotypic single-domain antibody (VHH) was selected from a naïve alpaca VHHs library to serve as a surrogate for CIT hapten. The phage-displayed VHH was used as a signal-amplification carrier to develop an indirect competitive phage enzyme-linked immunosorbent assay (P-ELISA) for the sensitive detection of CIT. The half-inhibition concentration (IC50) of P-ELISA was 10.9 μg/kg, which was 9-fold better than that of conventional ELISA (IC50=102.1 μg/kg). Results on P-ELISA analysis of naturally contaminated samples were also consistent with those obtained by conventional ELISA. In conclusion, the proposed P-ELISA demonstrates the potential use of phage-displayed anti-idiotypic VHH as surrogate for small molecules and signal-amplification carrier to improve assay performance for more sensitive analyte detection in food safety monitoring.