A multimode optical fiber includes a core and a cladding covering the core. The core has a radius in a range of 23.75-26.25 μm. A refractive index profile of the core has a graded refractive index distribution with a refractive index distribution index α in a range of 1.80-1.89. The core has a maximum relative refractive index difference Δ1% in a range from 1.0% to 1.15%. The multimode optical fiber has a fusion loss less than or equal to 0.08 dB. The multimode optical fiber is applied in a middle-and-long distance distributed temperature-measuring system, and a temperature-measuring distance of the system reaches 10 km to 27 km. The system includes a pulsed laser light source, a wavelength division multiplexer, an avalanche photodiode, a data acquisition device, an upper computer, and the temperature-measuring multimode optical fiber.