急需帮助，从美国回国要在加拿大转机需要过境签证吗？

如果，你的机票买的时候是美国---中国。那么这个航班不论在任何国家的转机，你都无需考虑过境签证、转机行李的问题。因为你转机也只是在机场内转机，你出不了机场的。
如果你自己买票，先美国--加拿大，加拿大---中国。那就需要签证了。

在加拿大转机如果不出机场的话是不需要加拿大签证的，可以放心。

Passengers arriving from the United States
In addition to the conditions above, when travelling from the United States, nationals from approved countries must meet the following conditions to qualify as a TWOV passenger:
They arrive in Canada from the United States on a direct, non-stop flight on an approved airline and proceed to the CBSA area for clearance or remain in an isolated transit area while waiting for a connecting flight (passengers that leave the isolated transit area must go to the CBSA area for examination);
They are not under a removal or deportation order;
They had a valid U.S. visa to enter the United States and have not overstayed the period of time they were authorized to remain in the United States;
They are destined for a third country, including their country of nationality, and have the required travel documents to enter that third country as well as the required travel documents for any countries they may transit through; and
They hold a confirmed onward ticket for a flight that will leave Canada on the same travel day (no layover or overnight stay is permitted).
They may be asked to fill out a TWOV card for statistical purposes.

以上信息来说加拿大CIC
http://www.cic.gc.ca/english/department/twov/travellers.asp