As we all know, as one of the rarest and most precious metals, gold, like silver, is one of the few precious metal varieties that can perfectly combine commodity attributes, monetary attributes and financial attributes. The relationship between gold and silver can be described as husband and wife or brotherly relationship, which is far from being comparable to other precious metals or commodities. This conclusion can be drawn from the high correlation between the physical properties, chemical properties, historical mission and the most intuitive trend of gold and silver. Gold investors have heard the saying: when the US dollar index goes up, gold will fall, and when the US dollar index goes down, gold will go up. In other words, the trend of gold and the dollar is negatively correlated. So, what is the reason for this negative correlation? Both gold and dollar belong to reserve assets. Dollar and gold are the most important reserve assets, but they are different in nature. Dollars belong to paper money and have no value in themselves, but holding dollars can bring interest income. Gold is a precious metal, which has high value in itself. At the same time, it has the function of avoiding inflation and risks, but holding gold can not bring interest income. Therefore, investors will choose between the two according to the political and economic situation. The trend of the US dollar is positively related to the US economic situation, while the trend of gold is negatively related to the global economy. The United States is the largest economic power, and its economic development will have an important impact on global economic development; The price of gold is generally inversely proportional to the world economy. When the American economy is prosperous, the global economic situation is usually optimistic, which leads to a strong dollar and weak demand for gold. On the contrary, when the American economy is weak, the global economic situation is usually not optimistic, which will lead to a weak dollar and strong demand for gold. Gold is priced in dollars. From the perspective of gold demanders, when the dollar depreciates, that is, the dollar index falls, investors who use other currencies, such as the euro, will find that when they buy gold with the euro, the same amount of money can buy more gold, thus stimulating demand, leading to an increase in demand for gold, and then pushing the price of gold higher. On the contrary, if the US dollar index rises, the price of gold will become more expensive for investors who use other currencies, thus curbing their consumption, and the decrease in demand will lead to a decline in the price of gold. From the perspective of gold producers, most of the gold mines are outside the United States, and the dollar index has had a certain impact on the interests of gold producers. Because the production cost of gold mines is calculated in domestic currency, and the price of gold is calculated in US dollars, when the US dollar index falls, the domestic currency returned by exports will decrease, and the profit will decrease, thus hitting the enthusiasm of producers, eventually leading to the decline of gold production, and the reduction of supply will inevitably raise the price of gold. From the historical trend of gold and dollar index, we can find that the two indexes will show the same trend, because the dollar and gold are hedging tools. When the world political situation changes or the economy is unstable, investors will choose hedging tools to avoid investment risks. The demand for gold and dollar as hedging tools increases at the same time, which sometimes makes both gold and dollar indexes rise, thus forming the same trend of dollar index and gold price. To sum up, the US dollar index is negatively correlated with gold and silver in the general direction, but the political and economic crisis and safe-haven demand sometimes lead to the same trend of gold and silver and the US dollar index, but it will not break the overall negative correlation.