A portable biosensor with high sensitivity is proposed, and the biosensor is based on a micro/nano-fiber coupler. When the biosensor is used in label-free biodetection, its sensitivity and reusability are studied. A fiber coupler with the waist diameter of 3 μm is fabricated by the fused biconical taper method, and then the coupler is used in refractive index detection experiment. A refractive index sensitivity of 1402.3 nm/RIU is measured in experiment. The detection results are fitted, and the correlation coefficient of the fitted curve is 0.99459.The calculated results of the weak coupling model of the micro/nano-fiber coupler are in good agreement with the experimental results. The detection limit of 2 pg/mL is obtained when the micro/nano-fiber coupler is used to detect goat immune globulin (IgG) antigen. The reusability of the biosensor is verified by 10 dissociation experiments. These experiments demonstrate that the micro/nano-fiber coupler can obtain a high detection sensitivity in the label-free biosensing and has good practical value.