Over the past decades, fiber-optic temperature sensors have received considerable attention due to their advantages of compact size, anti-electromagnetic interference, and low cost[1–4]. Such sensors have been widely applied in various fields, including environmental protection, food storage, health care, and industrial design. With the development of fiber-optic sensing technology, various temperature sensing structures have been proposed, like fiber Bragg gratings (FBGs)[5–7], Mach–Zehnder interferometers (MZIs)[8–10], Michelson interferometers (MIs)[11,12], and Fabry–Perot interferometers (FPIs)[13–15]. However, most of these sensing schemes are passive, resulting in low signal-to-noise ratio (SNR) and resolution. In recent years, benefiting from the narrow bandwidth, high resolution, and high SNR, fiber laser sensing schemes have been widely explored. Mandal et al. proposed a fiber-optic laser sensing probe for temperature sensing with the sensitivity of 12.01 pm/°C[16]. Gonzalez-Reyna et al. described a fiber ring laser used for temperature sensing where the FBG was used as the sensing head, and the temperature sensitivity was 18.8 pm/°C[17]. In order to improve the sensitivity of temperature sensors, Shi et al. proposed a fiber ring laser used for temperature sensing, in which the Sagnac loop served as the sensing head and the temperature sensitivity reached 1.739 nm/°C[18]. The demodulation system of these temperature-sensing schemes relies on expensive optical equipment, and the sensitivity and resolution of the sensing systems are limited. The beat frequency demodulation system is considered a fantastic method to realize the sensor’s demodulation, and it has attracted considerable research attention, since measurement in the electronic domain enjoys high speed, better resolution, and lower cost due to matured electronic technology. The beat frequency signal (BFS) is generated when the output optical signal of the fiber laser is transformed into an electrical signal by the photoelectric detector (PD). Therefore, the change in optical signal generated by the external environment will lead to a change in BFS. Consequently, tiny variations in the optical domain will lead to significant changes in the electronic domain. Recently, Yin et al. demonstrated a multilongitudinal-mode (MLM) fiber laser combined with the beat frequency demodulation system used for temperature sensing; the sensitivity of the temperature sensor reached 10.24 kHz/°C when the monitored BFS was at 1581.7 MHz[19]. Huang et al. reported a temperature sensor utilizing a multiplexed MLM fiber laser with the longitudinal mode beat frequency demodulation system, and the sensitivity was approximately [20]. Yu et al. described a temperature-sensing system by employing a polarimetric MLM fiber laser sensor, in which the temperature sensitivity reached [21]. However, the sensitivity of the sensor using normal fiber lasers for beat frequency was limited due to the limited number of longitudinal modes of normal fiber lasers and low SNR of the BFS at high frequency. Compared with normal fiber lasers, passively mode-locked fiber lasers have the advantages of more longitudinal modes and better SNR of BFS at high frequency, which shows great potential for sensing applications.