In the "Xuanwumen Change", Li Shimin killed his eldest brother Li and his younger brother Li Yuanji, and forced his father Tang Yuan to abdicate, becoming the second emperor of the Tang Dynasty. It can be said that Li Shimin's throne is stained with the blood of his own blood relatives. When Li Shimin became emperor, he made his eight-year-old son Li Chenggan a prince. In 642 AD, the Prince almost staged the "Change of Xuanwu Gate", but this incident was discovered by Li Shimin in time, which avoided a tragedy in which the royal family killed each other.