偶觉得是不可以滴.........

似乎是在入海关时移民官员给你的期限内可以多次国境，比如说你去加拿大的理由是开会，那移民官员给你的居留期限就是开会这段时间，在这段时间内你可以跑回美国来吃顿饭再回去，但是等你开完会回到美国就不能再用这个签证了

谢谢楼上几位的回答。我刚才给一个朋友打电话，她的回答也大概是这样。 谢谢大家！

谢谢trueblass，我又去mit看了一下，应该是半年有效，从第一次入境章开始。据说有手写自第一次入境，多久内可以无限次过。一些限制条件，我转过来：

这是从备忘录中的LINK抄下来的;
National CHINA (PEOPLE'S REP.) (CN)                        
Destination CANADA (CA)    

Passport (must be valid for period of intended stay) required.
Passport may bear Canadian "Visa Office Date Stamp" (size:
26 sq. mm./1 sq. inch). This stamp is not a visa.

Visa required.

If entering from the U.S.A. with a used single entry visa, the
visitor may re-enter without obtaining a new Canadian visa,
provided the I-94 from the U.S.A. and the Canadian port stamps
are still valid.

还有这段：


"A single entry visa allows you to enter Canada only once. This visa may also
be used for repeated entries into Canada from the USA or St. Pierre and
Miquelon provided you do not enter another country. Such entries must occur
within the time validated for your stay in Canada.(e.g. You may be given six
months from the date you enter Canada at the Port of Entry) "
http://www.cic.gc.ca/english/applications/guides/5256E2.html