Passive harmonic mode-locking（HML）technology, as an effective method for rapidly increasing the output pulse repetitionfrequency of fiber lasers, significantly improves the output pulse repetition frequency of fiber lasers. This technologyachieves high-order harmonic output by integrating parameter adjustable saturable absorbers inside the laser, thereby significantlyincreasing the pulse repetition frequency. This paper summarizes several commonly used types of saturable absorbers in passiveHML fiber lasers, elaborates on their technical principles and advantages, and analyzes the latest technological progress in thisfield at home and abroad. Finally, this paper points out the future development trend of passive harmonic mode-locked fiberlasers.