Abstract:Like many countries in the world, the United States is a country of exerting the power of mass media. It strives to use the mass media to influence the world. It is not easy to calculate to what extent the mass media can set the public agenda and how much they can affect the public opinion. But when we look back, and especially when examining the media exposure of terrorism, we can see clearly that the sense of terror amongst the public was created by the mass media rather than by the terrorists alone. The mass media are never neglected on such occasions as the event they reported has been shown in all forms such as radio, television, newspaper, magazine, etc. They give the public the first impression.

　　　Therefore, this thesis tries to find what a role mass media play in setting the political agenda and forming public opinion in the United States. As we know, the media play a pivotal role in affecting elections, monitoring the government, public communication, etc. On the other hand, the power constraints to the U.S media have certain limitations; this paper is also intended to analyze the reasons for the limitations of the media constraints. 

Key words: mass media; power; news selection; agenda setting

摘要:像世界上许多国家一样，美国也是“传媒力量上的国家”，它时刻在努力用传媒力量影响世界。很难估计，媒介在何种程度上可以设置议程并多大程度上影响大众舆论。但当我们回顾历史，特别是在研究媒体曝光的恐怖主义时，我们可以清楚地看到，在公众间产生的这种恐怖实际是由于大众传播媒介所致，而不单单是恐怖分子。被报导的事件通过各种主要形式（如广播，电视，报纸，杂志等）给公众形成第一印象，在这种情况下，媒体永远不可被忽视。

　　　本文将着眼于大众媒体在设定政治议程和形成舆论中所扮演的角色。媒体在影响选举，监督政府，设定议程，沟通民众等权力制约方面具有举足轻重的作用。与此同时，美国媒体在权力制约方面也有一定的局限性，本文还分析了媒体权力制约局限性的原因。

关键词：大众传媒；权力；新闻筛选；议程设置