The subject matter of stock index futures trading is the stock index. Stock index is an indicator to measure the overall performance of the stock market, which can reflect the ups and downs of the stock market, market sentiment and investors' expectations for the future development of the market. In stock index futures trading, investors do not buy and sell the actual stocks directly, but buy and sell stock index futures contracts to gain income or take risks. The subject matter of stock index futures trading is the stock index. In stock index futures trading, investors can invest by buying (long) or selling (short) stock index futures contracts. If investors think that the stock index will rise, they can buy stock index futures contracts and sell them for profit when the stock index rises; If investors think that the stock index will fall, they can sell stock index futures contracts and buy and close positions when the stock index falls. Compared with stock trading, stock index futures trading has leverage effect. Investors can control a certain value of stock index contracts by paying a small amount of margin, which can amplify the investment income, but it also brings corresponding risks.