Recently, pay-for-answer model has been evolved from the traditional free-based social question and answer platform (Q&A), and more and more users have been involved in the payment-based social Q&A activities. This paper investigates the impact of user identity on the answerers' switch behavior from free- to fee-based knowledge Q&A platforms. First, the study presents a literature review on the influencing factors of participants’ motivations and behaviors in online knowledge Q&A platforms. Then, we crawl the answerers’ behavioral information in both free- and fee-based knowledge Q&A platforms, and conduct some preliminary analysis. Furthermore, the propensity score matching method has been employed to examine the impact of user identity on the answerers' switch behavior, and the robust test has been conducted and verified. The research findings show that the certification of answerers‘identity does not have a significant impact on the answerers' switch behavior, while the title of excellent answerers has a positive significant impact on the answerers' switch behavior.