Capital structure fluctuation (CSF) alters financial risk and capital cost, which is an important factor affecting firm value. But it still needs further research to make out which factors affect CSF. In this article, based on the financial data of A-share listed firms from Shanghai Security Exchange and Shenzhen Security Exchange(2003—2012), we try to disclose the effect of firm scale and the expansion of financing constraints on CSF. We find that financing constraints have a positive relation with CSF; firm size has a negative relation with CSF. Firm scale growth can offset the driving effect of financial constraints on CSF; the effect of firm scale and the expansion of financing constraints affecting CSF in a state owned firm is less than that of a private firm.