sdfsdfsdff is correct. 由于北美自由贸易协议, 加拿大公民不需要签证. 只是在入境口岸申请入境. 也没有30天的限制. 你去中国旅游回来, 在入境口岸只要出示I-797 原件就行. But it's also better to show two-months stubs, employer letter, etc.