Tetrabromobisphenol A (TBBPA) is a flame retardant and has become a widely concerning environmental pollutant. An ultrasensitive nanobody-based immunoassay was developed to monitor the exposure of TBBPA in sediment. First, the anti-TBBPA nanobody was fused with nanoluciferase, and then a one-step bioluminescent enzyme immunoassay (BLEIA) was developed with high sensitivity for TBBPA, with a maximum half inhibition concentration (IC50) at 187 pg/mL. Although approximately 10-fold higher sensitivity can be achieved by this developed BLEIA than by the classical two-step ELISA (IC50 at 1778 pg/mL), it is still a challenge to detect trace TBBPA in sediment samples reliably due to the relatively high matrix effect. To further improve the performance of this one-step BLEIA, a C4b-binding protein (C4BP) was inserted as a self-assembling linker between the nanobody and nanoluciferase. Therefore, a heptamer fusion containing seven binders and seven tracers was generated. This reagent improved the binding capacity and signal amplification. The one-step heptamer plus BLEIA based on this immune-reagent shows an additional 7-fold improvement of sensitivity, with the IC50 of 28.9 pg/mL and the limit of detection as low as 2.5 pg/mL. The proposed assay was further applied to determine the trace TBBPA in sediment, and the recovery was within 92-103%. Taking advantage of this heptamer fusion, one-step BLEIA can serve as a powerful tool for fast detection of trace TBBPA in the sediment samples.