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Harmonic mode‐locking (HML) is an important technique enabling the generation of high‐repetition‐rate ultrashort pulses. Using an emerging time‐stretch dispersive Fourier transform technique, the experimental observation of the entire buildup process of the passive HML state in an ultrafast fiber laser is reported here. It is unveiled that the whole process of HML buildup successively undergoes seven different ultrafast phases: raised relaxation oscillation, spectral beating behavior, birth of a giant pulse, self‐phase‐modulation‐induced instability, pulse splitting, repulsion and separation of multiple pulses, and a stable HML state. It is observed that the multiple HML pulses originate from a single‐pulse splitting phenomenon and a remarkable breathing behavior occurs at an early stage of the HML buildup process. The numerical results confirm that the effects of dispersive wave, gain depletion and recovery, and acoustic wave play key roles in the earlier, middle, and later stages of this HML buildup process, respectively; as well, the acoustic resonance in the single‐mode fiber stabilizes the final HML state of lasers. [ABSTRACT FROM AUTHOR]