In stock index futures trading, margin is the fund used for settlement and performance guarantee. Investors who participate in stock index futures trading, whether as buyers or sellers, need to pay corresponding deposits according to their positions. Margin can be divided into two parts: trading margin and settlement reserve. The trading margin is calculated according to a certain proportion of the contract value of the position, which is occupied by the contract and may not be used for other purposes. The part of the futures margin account that exceeds the trading margin is called the settlement reserve, which investors can freely control, and it is also called the available funds.

For the sake of risk control, futures companies will generally increase the minimum margin ratio stipulated by the exchange by a few percentage points. For example, when the margin ratio stipulated by the exchange is 15%, the margin required by the futures company may be 18%. Investors are reminded that even if the number of investors' positions has not changed, the amount of trading margin required may change. This is because, first, the fluctuation of contract price will lead to the change of contract value, so the requirements for trading margin will also change. Second, more importantly, the trading margin ratio is not fixed and may be adjusted.