The ascension of Emperor Xuan as the Huang Tai Shu(皇太叔) to the throne was a very exceptional phenomenon not just in the Tang Dynasty but also in the whole history of China. The existing research has studied in the perspective of the political participation of the Eunuch in the late Tang Dynasty, and as a result it was understood that the Eunuch who was deeply involved in the decision of the throne was to crown Emperor Xuan as the emperor for the purpose of seizing future power. However, should the situation prior and post to the accession of Emperor Xuan be considered, there is a possibility of another separate background that could have applied regarding the ascension. For this, there is the need for an understanding of the accession of Emperor Xuan in the continuum of succession to the throne. If we were to trace the aspect of succession to the throne that led from Emperor Xian, Emperor Mu, Emperor Jing and to Emperor Wu, it can be seen that the notion by which the Eldest son(長子) inherits the throne has been actively utilized. Furthermore, several coups and rebellions that occurred in the process also confirm the use of the concept of succession by the eldest son. Should we see this as the setting, Emperor Xian being able to ascend the throne as being the uncle of an emperor was because the cause identified by experience accumulated to that day or in other words, the concept of the eldest son to be the next in line to the throne was in common use. Interestingly, however, this aspect emerged while the Eunuch were directly involved in the succession of the throne. As the original explanation of the accession of Emperor Xuan shows, despite the logical assumption that the selection of a young heir would help the so-called eunuch political power, in reality, we are able to see that the selection of heirs to the throne with the justification as the Eldest son was applied in the late Tang Dynasty. This is in stark contrast to the situation in which the Eunuch in the late Han Dynasty took control by crowning an underage emperor themselves. In the latter part of the Tang Dynasty, it is also understood that the Eunuch attempted to strengthen their power on the basis of enthroning emperors that are of age.