樂府「古辭」的界定一直存在較大爭議，但學界對《宋書》所收古辭皆無異議。本文通過詳細考察《宋書》所收古辭的內外部特徵，從而歸納出古辭的一般特徵，即古辭乃兩漢產生的、在兩漢魏晉由聲樂系統而入器樂系統歌唱的民歌。首先，篇題名或曲調名與古辭內容有一定聯繫；其次，具有入樂的特點，這種入樂的特點是指由聲樂系統而合器樂系統，其入樂又是一個長期演變的過程，即在兩漢魏晉諸朝，將漢樂府民歌逐步由聲樂系統過渡到器樂系統；最后，還具有民間文學性的特性，這種民間文學性對古辭的創作群體和流傳方式作了規定和限制。

The definition of Yuefu Guci has created considerable controversy. However, the words of the Song Book, which was written by ancient scholars, are received without objection. This paper reports a detailed investigation of the Song Book, which possesses the internal and external features of the Yuefu Guci. In addition, this paper summarizes the general characteristics of the Yuefu Guci, which was based on the instrumental system of folk singing and was produced during the Han, Wei, and Jin Dynasties. The article title or name of the song (accompanied by the contents of ancient speech) have connections to particular music features, which are characterized by the evolutionary process of the vocal system being combined with the instrumental system of music during the Han, Wei, and Jin Dynasties. Particularly, Han folk songs gradually transitioned to the instrumental system from the vocal system. The influence of folk literature on the formation of groups, as well as the spread of ancient speech, resulted in regulations and restrictions.