留学生如何签证美国？
2019年5月3日更新 感谢 @王雨磊 和 @张哲轩 的指正，我漏掉了F-1的初始I-20表（Initial I-20），它应该是在学生第一次入学的时候从学校获得的表格。 对于大部分在美国上本科和硕士课程的人来说，需要办理J-1签证以合法留在美国。
关于J-1签证的要求可以在DS-260表格中查看，填写该表格时需要选择你的签证类型，其中就包括J-1 (Fellowship/Scholarship)或J-1 (Profession). 在美国攻读博士课程的留学生可以申请J-1(Professional)签证；而申请硕士以上学位的学生则必须申请J-1(scholarship / fellowship)的签证.
下面我来分别解释这两者的区别以及他们各自所需要满足的条件和要求：
首先我们来讨论 J-1(professor) vs S-3 visa ：
两者都是J-1签证类别，不同的是前者只适用于访问学者，后者则是用于申请去美国攻读PhD或者Master Degree。在签证面谈时，你被问到的主要的问题将围绕着为什么你去读PHD而不是做访问学者来进行。因此，如果你在面谈时被问到“Why don't you apply to be a visiting scholar instead？” 请一定要小心作答！
那么两者的具体要求是什么呢？
访问学者的基本要求如下： 需要注意的是，这些条件仅仅只是最低标准而已，各个学校会根据自己的要求进行进一步的筛选。
例如有的机构要求学生拥有20篇以上的论文发表，或是必须达到每年3万元以上的工资水平等。 至于S-3 visa的申请，请看这里： URL#section7
虽然这个签证允许你在美国接受PHD的学习，但值得注意的是，它的有效期只有一年。所以你必须要在一年内完成所有的学业，否则的话你将失去合法留在美国的资格。此外由于这是一种留学签证，你也必须遵守与留学相关的所有规定、缴纳学费等等。如果你有任何疑问，请向学校的国际学生事务办公室咨询。
接下来我们谈谈最普遍也最容易申请的 J-1(fellowship / scholarship ) visa：
J-1 (Scholarship or Fellowship) is the most common type of work permit， issued by an American organization and it allows you to stay for as long as your program lasts but not longer than six years on average after finishing school. In order to qualify for this Visa， you need to demonstrate that：
Your host university has been approved by both the U.S. Department of State and the USCIS. This should be clearly stated in any letter that they send to you. If there are other universities listed next to yours， then only those institutions can sponsor you． The application process takes approximately one year， so make sure you start early！ You may also check if their sponsoring schools have been approved at：
You will likely receive all documents electronically through email from US Embassy or Consulate General’s website where you plan to submit your applications. Please download them， save them into a folder with your name as well as your passport number attached. And remember these materials must always be kept safe because you might be asked about them when applying for visas at overseas embassies and consular offices！ Also before submitting your completed applications， please review carefully Section A “Application for Post Study Work Permit Program Form" found here：
In addition to being sponsored，you also need to meet some requirements