Highlights •The work provides a novel method with high sensitivity and reproducibility for in situ detection of dissolved trace furfural in transformer oil.•Flower-like silver nanoparticles with large specific surface area and strong adsorption capacity are successfully synthesized by the chemical reduction method combined with electroless silver plating.•Flower-like silver nanoparticles exhibit high SERS activity, uniformity, reproducibility and stability.•The SERS assay is applied for the detection of multi-analytes in real samples.