In fact, I did not know the Chinese stock market at all, I worked for one stock trading company in New York. So, I try to explain these in English if it could help you to understand it. In a margin account, you do not pay in full for stocks that you buy or sell. Under current Federal Reserve Board rules, you must deposit half of the stocks market value at the brokerage firm when you establish the position. The remainder you borrow from the brokerage house. If you have $3000 to invest and use cash account, you can buy only $3000 worth of securities. If you buy the securities on margin, you can purchase $6000 of securities. To remind you, you must pay for the interest you used from brokerage, and when you buy or sell shares on margin, the certificates are always held in street name.A demand from a brokerage firm for a investor to deposit cash in a margin account is called margin call.