Low-cost and miniaturized biosensors are key factors leading to the possibility of portable and integrated biomedical system, which play an important role in clinical medicine and life sciences. Random lasers with simple structures provide opportunities for detecting biomolecules. Here, low-cost biosensors on fiber facet for label-free detecting biomolecules are demonstrated based on a plasmonic random laser. The random laser is achieved resorting to a self-assembled plasmonic scattering structure of Ag nanoparticles and polymer film on fiber facet. Refractive index sensitivity and near-surface sensitivity of the biosensor are systematically studied. Furthermore, the biosensor is used to detect IgG through specific binding to protein A, exhibiting the detecting limit of 0.68 nM. It is believed that this work may promote the applications of a plasmonic random laser bio-probe in portable or integrated medical diagnostic platforms, and provide fundamental understanding for the life science.