A novel microfiber nested ring with knot resonator (MNRKR) is proposed and demonstrated. A mathematical model to describe the principle of the tunable feature of MNRKR is illustrated. The improvement of the output spectra is achieved by changing the transmission coefficient and the length of the feedback waveguide. We theoretically and experimentally demonstrate that the free spectral range (FSR) can be doubled by constructive interference without reducing the circumference of microring. Thus, this all-fiber optical device has the potential for achieving a large measuring range of the optical sensor.