中文摘要：

股权分置改革的完成并不能消除上市公司大股东与中小股东之间的利益冲突。在全流通时期，大股东理性参与上市公司的定向增发、资产注入及其后的股份减持很可能诱致股市的过度波动。对这种股市过度波动所引致的股市脆弱性机理我们进行了模型推导和因子分析。分析表明，牛市中，大股东参与定向增发、资产注入会导致牛市的助涨。而当熊市来临时，大股东会停止资产注入并减持股份，进而起到熊市助跌的作用。针对由此引起的股市脆弱性，监管部门应采取相应的监管措施。

英文摘要：

The accomplishment of stock split reform can＇t delete the conflicts between the major and minor stockholders. In the full circulation period, major stockholders may rationally take part in the private issue and inject their assets into the listed company, and sell out some shares, which can lead to the over-fluctuation of stock market. The paper models the stock market＇s fragility by this over-fluctuation, and makes in-depth analysis of factors. The analysis shows that private issue and assets injection lead to dramatic rise in the bull market while in the bear market major stockholders will stop assets injection and sell out part of their share, leading to Sharp fall in price. It is suggested that the supervisory agents should take corresponding action to oversee the stock market fragility caused by major stockholders＇ rational actions.