The removal of the ban on margin purchase and short sale in 2010 not only introduces a short-selling mechanism for Chinese market investors， but also provides a leverage amplification mechanism for speculative traders． Different from the previous perspective of institutional change， this paper， from the micro-behavioral perspective， studies the credit account investors’trading behavior and its predictability． Based on the sample data from December 2011 to August 2015， it is found that the credit account investors increase their margin purchase following the positive return of the stock， and increase the short sale after the negative return． Further studies show that intensified margin purchase can predict positive future returns， while intensified short sale can predict negative future returns． In addition， the margin purchases have a significant positive impact on the future stock price collapse， while the short sale can reduce the possibility of stock price collapse． Our study reveals the micro behavior of margin purchase and short sale， and also provides theoretical basis for further reforms．