The effect of water mist on the extracted dentin ablation by Q-switched Er:YAG laser is investigated, and the influence of water mist on the ablated dentin microstructure is studied. The Q-switched Er:YAG laser with 62 ns pulse width is used to ablate the human dentin at the repetition rate of 3 Hz, the number of pulses is 10, and the energy range is from 4 mJ to 70 mJ. Different flow rates of water mist (0, 4, 9, 13 mL/min) are applied during the experiments. 10 dentin samples are ablated under each experimental condition. A scanning electron microscope is used to observe the microstructures of ablated holes. The experiment result shows that the increase of flow rate of water mist makes the Q-switched Er:YAG laser ablation threshold to dentin rise, and the dentin is protected from mechanical damage. The results are guided for acquiring microstructures during dentin ablation with Q-switched Er:YAG laser and practical treatment in dentistry.