Visitor Visa Exemptions

Many people do not require a visa to visit Canada. These include:
* persons lawfully admitted to the United States for permanent residence who are in possession of their alien registration card (Green card) or can provide other evidence of permanent residence;

拥有美国永久居民(绿卡)身份文件、或能提供其它文件证明身份的美国永久居民(绿卡持有者) 不需要签证进入加拿大。然而、如果你已经在美国之外很长时间，在进入加拿大前你必须先确认你的永久居民状态。
http://www.canadainternational.gc.ca/china-chine/visas/beijing-pekin/visiting-visiter.aspx?lang=zh-cn&view=d#a1