Sporadic tonal conversion refers to the phenomenon that a small amount of characters belonging to one tone in Old Chinese evolved into another tone in Middle Chinese. There are two kinds of documents which can be used for the identification of the tone of a character in Old Chinese, i.e., verse and phonetic loan characters. In this paper, we made in-depth exploration of the phonetic loan characters from the inscriptions on bamboo slips, old annotations of classics and verse to outline the process of sporadic tonal conversion. And the reasons why this conversion happened were also discussed.