Rapid and high‐sensitive detection of mycotoxins is believed to be of vital importance in assuring food safety. In this study, we developed a novel fluorescence immunochromatographic sensor (ICS) for the mycotoxin of zearalenone (ZEN) in cereals. This was done by using a black phosphorus‐Au nanocomposite (BP‐Au) as the 2D quenching platform. Herein, gold nanoparticles (AuNPs) were directly reduced on the surface of BP nanosheets (BPNSs) to form BP‐Au nanocomposites, showing higher fluorescence (quantum dots, λ _Em = 525 nm) quenching efficiency compared to the BPNSs and AuNPs. The fluorescence quenching efficiency of the prepared BP‐Au nanocomposite reached 73.8%, which was 1.73‐fold and 1.44‐fold higher than AuNPs and BPNSs, respectively. The density functional theory was also successfully used to explore the formation mechanism of the BP‐Au nanocomposite. By introducing the quantum dots/BP‐Au signal/quencher pair, a high‐sensitive fluorescence quenching ICS (B‐FICS) was developed for the detection and discrimination of ZEN with the limit of detection of 0.1 μg/l in pure working buffer. This was 2.5‐fold more sensitive than AuNPs‐based FICS (A‐FICS). The B‐FICS was successfully applied in real cereals detection with the sample limit of detection of 2 μg/kg. The successful construction of B‐FICS offers a novel method for a rapid and high‐sensitive detection of ZEN in cereals. It also provides a new practical application of 2D BPNSs in food safety sensing.