The visa number can be found on the Visa sticker on your passport page. The number can be 8 or more characters long, and in RED color, located towards the bottom right hand quarter of the Visa sticker. Be advised that the "Control Number" is not the Visa number. If you mean a non-immigrant visa such as a B1, B2, F1, H1B, then it's on the visa itself. The visa would be the nice-looking laminated sticker (the new ones, at least) on your passport, and the number would be a long number on the top of the visa. If you lost it, there's going to be some minor problems obtaining the number, but you would have to contac the Dept. of State or the consular section where you obtained the visa. If you're talking about an immigrant visa, an A-number may or may not be assigned to you if you're still waiting for the petition to be approved. Generally, this would not be termed a "visa number". Check out any I-797 Notice of Action mailed by USCIS. The 9 digit number (sometimes 8 digits since the first number is assumed to be 0) would be above the beneficiary name (A *** *** ***). If the number's not there, then there's none assigned. Depending on why you need the "visa number", you can use the receipt number as proof that there's a pending petition (the receipt number is 13 characters long and starts with three letters like EAC, WAC, or MSC). For Non-Immigrant Visa, Visa number is the RED Number on the Visa itself. This answer is from USCIS.
上述摘自人家官网http://www.uscis.gov/即美国公民及移民服务局。它是美国政府处理在美外籍人士事务的主要负责机构。911以后的2003年1月，原美国移民局（INS）与国土安全部（DHS）合并成立了美国公民与移民服务局（USCIS）。

谢谢，我正在线填写，卡到这个位置不知道如何解决，十分感谢。

填写你前年签证的编号，签证页右下方的红色数字。

贴在护照上签证纸上右边一串红色的数字或字母组合

签证页上有写明的。

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【签证类型】：美国个人旅游签证
【入境次数】：多次
【停留时间】：180天（具体海关批）
【有 效 期】：10年有效
【办理时长】：12-20个工作日（面试后8-10工作日）
【是否面试】：是
【送签领区】：广州领区/上海领区/北京领区/成都领区
【受理范围】：不分领区，全国各省份颁发护照就近办理。