Price theory is the core of microeconomics.In the complete market economy,demand and supply are the two powers that decide the price.Stock index futures are futures based on the overall changes of the sample stocks.The special mechanism of selling short decides the key to the price determinism of futures market is how to understand the relationship between supply and demand of futures contract.The author regards hedgers act as the supply part in futures market，the amount of whose supply is decided by the cost of the hedge.Speculators and arbitragers both are the demand parts in futures market purely，the amount of whose demand is decided by risk premium.The equilibrium price of the futures market is decided by the balance point of supply and demand.