After the Qin dynasty unified the currency, later dynasties also used gold as the currency, but gold was too rare and the cost of ancient smelting technology was too high, so the value of gold was very high.

The currency in circulation is mainly copper coins, supplemented by gold and silver. The smelting technology of copper and iron is very complete, and the reserves are relatively large, but iron is mainly used as agricultural tools, machinery and weapons. The value of copper is higher than that of iron, so it bears the responsibility of currency.