签证管得是你进出美国，所以是一年进出

I20是你的学生身份，所以是84个月

Yes, everybody's vias is for one year. You don't renew your visa as long as you are in the U.S.

But once you go back to china, you have to renew it in Beijing

签证只是让你合法入境。

有I20说明身份的话，你在美国呆着就没有问题。

只是如果你想中途短期再去别的国家，如果签证过期的话，好像要回中国，先办美国签证，

为你能合法去别的国家，但再入境的时候你的美国签证已经无效了。

Student(F/M)

How long may I stay on my F1 or M1 visa?

The Department of State (DOS) (travel.state.gov) is responsible for visa adjudication at U.S. Embassies and Consulates outside of the United States. The Department of Homeland Security (DHS) (http://www.cbp.gov/) determines admission into and conditions of stay in the U.S. at the port of entry. Please check carefully with official sources of information before applying for a visa or seeking entry into the United States.

When you enter the U.S. on an F1 or M1 visa, you will usually be admitted for the duration of your student status. That means you may stay for as long a period of time as you are a full-time student, even if the F1 or M1 visa in your passport expires while you are in America.

For example, if you have a visa that is valid for five years that will expire on January 1, 2005, and you are admitted into the U.S. for the duration of your studies (often abbreviated in your passport or on your I-94 card as "D/S"), you may stay in the U.S. as long as you are a full-time student.

Even if January 1, 2005 passes and your visa expires while in America, you will still be in legal student status. However, if you depart the U.S. with an expired visa, you will need to obtain a new one before being able to return to America and resume your studies. An F1 or M1 visa cannot be renewed or re-issued in the United States; it must be done at an Embassy or Consulate abroad.

http://seoul.usembassy.gov/f_stay.html