In recent years, with the rise of populism around the world, the traditional party system and democratic system have encountered very serious challenges. This article mainly takes the United States as an example to explore in depth how populism affects American political parties and democratic governance. First, the development history of populism in the United States since the 20th century is analyzed and introduced, focusing on the impact of Trumpism in American politics. Secondly, it mainly explores the impact of populism on the internal structure and policy orientation of the two major political parties, the Republican Party and the Democratic Party, and deeply explores the role of populism in splitting party unity and reshaping party ideology. In addition, it also focuses on the role of populism in democracy from both positive and negative aspects. Finally, this article examines the challenges of American populism to the development of American democratic systems and political parties, including challenges to the electoral process, judicial independence, media freedom and public trust. This article believes that strengthening civic education and promoting party reform can better cope with the impact of populism and enable the healthy development of the democratic system.