The main contract of stock index futures is the contract with the largest position/volume, which has become the main contract of stock index futures. The main contract changes with the change of market sentiment, sometimes it is a contract after one or two months, or it may be a contract after several months. The full name of stock index futures (SPIF) is stock index futures, which can also be called stock index futures and futures index. It refers to the standardized futures contract with the stock price index as the subject matter. The two parties agree to buy and sell the underlying index according to the size of the stock price index determined in advance at a future date, and settle the difference in cash after the expiration.