The future expectation of futures' reaction to spot, the quantitative performance of this expectation, is the price difference, in other words, the premium or discount, and the range of premium and discount, and their changes. The law of price difference is as follows: when the price difference is positive and becomes larger, it means that the premium is increasing, and futures are optimistic about spot expectations; When the spread is positive and small, it shows that the premium is reduced and the futures are cautious about the spot expectation; When the spread is negative and small, it means that the discount rate increases and futures are not optimistic about the spot; When the spread is negative and increasing, it means that the discount range is reduced and futures are no longer so pessimistic about the spot. The practical significance of spread or premium is as follows: spot price rises, spread increases and premium increases, indicating that futures prices rise faster than spot prices and are expected to be optimistic; The spot price rises, the spread narrows, and the premium falls, indicating that the futures price rises more slowly than the spot price, and it is expected to be cautious. Spot fell, spread widened and premium increased, indicating that futures fell slower than spot and were expected to stabilize; The spot falls, the spread narrows, and the premium rate decreases, indicating that the futures fall faster than the spot and the expectation is pessimistic; Of course, if the price difference is already negative, it will be even less optimistic, but discounts are rare and last for a short time. Most pessimism is reflected in the decline of the price difference. Spread and premium quantify the mood of funds and reflect the attitude of funds, which can not be ignored by participants engaged in securities investment or speculation.