Thermal effect in gain fibers is one of the major factors limiting the output power of high power lasers. Measuring the temperature of gain fibers with the distributed temperature sensing method is beneficial to protecting fiber lasers, and may provide a new approach to understand the underlying mechanisms of the mode instability (MI) and other nonlinear effects occurring in fiber lasers. Major technologies on distributed optical fiber sensing are introduced, with more focus on the applications of technologies, such as optical frequency domain reflectometry (OFDR), Brillouin optical time domain analysis (BOTDA), Brillouin optical frequency domain analysis (BOFDA), and Brillouin optical correlation domain analysis (BOCDA), in the measurements of temperature and stress in fiber lasers. Finally, the feasibility of some distributed sensing schemes is analyzed, which provides references for temperature measurement in high power fiber lasers.