A novel microfiber sensor based on supermode interference is experimentally demonstrated through tapering a section of weakly-coupled seven-core fiber, aiming to develop a quantitative detection method for the AKT protein in cells. The experimental results show that the tapered seven-core fiber with taper waist diameter of 16.5 μm has the refractive index (RI) sensitivities of 882.3, 2017.72, and 6780.7 nm/RIU at the RIs of 1.33, 1.37 and 1.42, respectively. And the wavelength shift of 0.44 nm has been observed for the AKT concentration of 0.5 ng/ml.