The invention provides a silicon-based refractive index sensor based on a cascaded U-shaped waveguide nested micro-ring. The sensor is composed of a silicon substrate layer, a silicon dioxide lower cladding layer, a monocrystalline silicon core layer and a silicon dioxide upper cladding layer which are stacked in sequence from bottom to top, wherein the output end of a first straight waveguide inthe monocrystalline silicon core layer is connected with the input end of a first U-shaped waveguide, the output end of the first U-shaped waveguide is connected with the input end of a second straight waveguide, the output end of the second straight waveguide is connected with the input end of a second U-shaped waveguide, and the output end of the second U-shaped waveguide is connected with the input end of a third straight waveguide; a first micro-ring resonant cavity is located between the first straight waveguide and the second straight waveguide on the inner side of the first U-shaped waveguide, and a second micro-ring resonant cavity is located between the second straight waveguide and the third straight waveguide on the inner side of the second U-shaped waveguide. Compared with a cascade micro-ring sensor in the same size, the sensor has the advantage that the sensing detection range can be doubled on the basis of improving the sensing sensitivity. The sensor has the advantagesof high sensitivity, large sensing range, facilitation of on-chip integration and the like.