Interchangeability was an significant phenomenon of Chinese char-acters in ancient texts that occured when the pronunciations of two characters were similar or equivalent. There are great amout of inter-changeable charaters in both the extant Chinese classics and the recently unearthed texts. By analying the interchangeability of Chinese characters, we can not only observe the development and usage of Chinese characters, but also acquire better understanding of the old Chinese phonology preserved in those characters. The interchangeabil-ity of characters in Qin Banboo Slips at Shuihudi is salient and complicated as well. In this article, I discuss this phenomenon and its priciples with reference to Shuowen and other recently unearthed docu-ments.