A novel SOI (silicon on insulator) temperature sensor based on a U-shaped waveguide-coupled single micro-ring is proposed in this paper. Refractive index and length of the temperature-sensing part will change as the temperature changes, leading to the shift of the output spectrum of the sensor. The transfer function of the U-shaped waveguide coupled with a single micro-ring is obtained based on the theory of coupling modes and transfer matrix method. And we have studied the output spectrum properties of the system from different temperature-sensing parts. Results show that the whole structure of U-shaped waveguide coupled with the single micro-ring proves to be the best temperature-sensing part as the output spectrum with no spurious modes and a deep extinction ratio of 31 dB. Compared with the traditional two waveguides coupled with a single micro-ring resonator, when the distance between the two coupling points of the U-shaped waveguide is an integer multiple of circumference of the micro-ring, the free spectrum range (FSR) can be expanded to 56 nm, the sensitivity can achieve 89.2 pm/℃, and the measurement range be 298720 K, achieving a high temperature measurement of SOI micro-ring resonator.