On 6 July 2018, the United States began to impose 25% tariffs on $34 billion of Chinese goods. The trade war between China and the United States was officially launched. It lasted for more than a year, during which the two sides experienced six mutual tariff increases, which significantly impacted a few fields in China and the United States. This paper selects the RMB exchange rate to the US dollar as the research object; and takes the return of the RMB exchange rate as the data basis for modelling and analysis, intercepts its return data in the five years before and after the US-China trade war, taking the time when the United States imposed six additional tariffs on China as a dummy variable, and analyses the data by using the ARMA-GARCH model to study the impact of the RMB exchange rate by this trade war and to make predictions development of the RMB exchange rate, which will then make suggestions for China's economic.