Research result: 26.7% mandibular wisdom teeth, having the highest point located lower than the 7th tooth CEJ (type c), the rest was type b; 93.3% of mandibular wisdom teeth having smaller bone width than the crown width (type ii), the percentage of crooked and tight roots was dominant (70.0%); all cases of underground mandibular wisdom teeth had a difficulty of 3 (58.1%) or 2 (41.8%).