Stock index futures is a kind of financial futures. Futures trading refers to a trading contract in which both parties agree to buy and sell a certain quantity and quality of the subject matter at a certain price by buying and selling futures contracts at a certain time and place in the future. The ultimate goal of futures trading is often not to transfer the ownership of the subject matter, but to avoid the spot price risk by buying and selling futures contracts. As the name implies, stock index futures are futures contracts with monetized stock indexes as the subject matter. The most important function of stock index futures is that investors can use it to hedge stock spot investment and avoid systemic risks. Investors here are relative to speculators. The purpose of investors' investment in stocks is to obtain long-term capital gains through the increase of the stock price with the substantial growth of listed companies, and to obtain investment gains through the distribution of listed companies over the years. They hope to avoid the losses caused by stock market fluctuations, especially the violent fluctuations caused by unexpected events. If there is no short selling mechanism, once the stock market fluctuates violently, there may be a chain reaction of investors selling stocks one after another, adding fuel to the fire and further aggravating the stock price decline. Stock index futures trading provides investors with a mechanism to avoid systemic risks in the market. The principle is that according to the same trend of stock index and stock price changes, the spot market and futures market of stock index do opposite operations to offset the risk of stock price changes. In this way, when the market may fluctuate, investors can preserve the value of their investment without selling the stock itself, thus stabilizing the market, protecting the interests of investors and maintaining the healthy development of the securities market.