Sensors for rapid and reliable detection of biomolecules are crucial for clinical medical diagnoses. Here, a rapid, ultra-sensitive, magnetic-assisted biosensor based on resonance Raman scattering at MoS@FeO composite nanoflowers is presented. Raman shifts and X-ray photoelectron spectra indicated that the composite was formed via Fe-S covalent bonds. Convenient magnetic separations could be performed because of the superparamagnetic FeO nanoparticles. MoS E and A Raman peaks were used as probe signals for anti-interference immunoassays. The probe unit of the immunoassay also included goat anti-human IgG molecules that were used as the target analyte. Au substrates coupled with the goat anti-human IgG were used as capture units to form sandwich biosensors. Because of the magnetic enrichment, the detection limit was improved by three orders-of-magnitude and the detection time was reduced from 1.5 h to 1 min. Sandwich biosensors using MoS@FeO nanoflowers as Raman probes could be very promising sensors for proteins, antigens, and other immunogenic biopolymers, as well as for corpuscular viruses and cells.