The information asymmetry among investors has negative influences on the market efficiency.This paper studies the impact of opening B-share markets to domestic investors on the information asymmetry.We decompose the bid-ask spread into liquidity and information asymmetry components,and then compare their differences in the pre-event and post-event periods.Our results show that the opening event enhances the market liquidity,but doesn't mitigate the information asymmetry among the investors.