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A novel multi-longitudinal-mode fiber ring laser temperature sensor based on the beat frequency demodulation technology is presented. The beat frequency between any two modes varies linearly with the temperature. It is experimentally demonstrated that the temperature can be measured with a sensitivity of 10.24 kHz/° C @ 1581.7 MHz between 21° C to 50 ° C, 7.42 kHz/° C @ 1175.6 MHz between 22 °C to 240 °C and 8.48 kHz/° C @ 1480.5 MHz between 100 °C to 1000 °C. This method offers a simple scheme for fiber temperature sensing by employing a low-frequency electrical frequency spectrum analyzer.