Stock index futures are both long and short in the whole market. Stock index futures adopt a two-way trading mechanism, with one-to-one correspondence between long and short positions, and the long and short positions must be equal. No matter at any time, for a single investor in the market, the long and short positions may be unbalanced, but from the perspective of the whole market, every short order is matched with a single and multi-single. If there is no counterparty, unilateral bulls and bears can't open positions. Therefore, the total position in the stock index futures market will not add additional pressure to the spot market.

When the price of stock index futures is significantly higher than the spot index, arbitrage and hedging operations can reduce the premium of index futures. When the stock index futures are significantly discounted relative to the spot index price, it is difficult to reduce the discount by reverse arbitrage (that is, short selling the spot and buying stock index futures) because of the limited scale of spot securities lending and the great difficulty in operation.

The operation mode of arbitrage is to buy the spot and sell the futures when the futures price is higher than the spot, and earn the expected annualized expected return of the difference; The operation of hedging is generally to hold the spot first. When it is found that the market is falling and it is too late to sell the spot, the risk of spot decline is avoided by short selling stock index futures. Although both arbitrage and hedging will short futures, they will also buy or hold spot, which has a neutral impact on the market.

For the speculative trading of stock index futures, the main operation method is to conduct long-short trading of stock index futures by judging the short-term trend of spot index. It is often seen in intraday trading that stock index futures fall before the spot, and then the spot index also falls; If the spot index does not fall, short speculators will quickly reverse the operation, making stock index futures rise before the index.

In China stock market, the main forms of short selling of stock index futures are hedging, arbitrage and speculation.