From 2013 to 2015, China lifted the ban on margin trading for designated stocks based on apublic ranking index. Using a regression discontinuity design that exploits the threshold rules, I find that margin trading eligibility causes the stock share turnover and prices to increase. Moreover, firms react to this speculative pressure by manipulating earnings and reducing long-term investment. These effects are stronger for firms that are more prone to investor short-termism ex-ante. Consistent with managerial myopia, marginable firms later experience a decline in operating performance. My results suggest that margin traders, as short-term speculators, pressure the manager to focus on current earnings and take myopic actions.