USCIS官方网站写的很清楚:
Does travel outside the United States affect my permanent resident status?
Permanent residents are free to travel outside the United States, and
temporary or brief travel usually does not affect your permanent resident
status. If it is determined, however, that you did not intend to make the
United States your permanent home, you will be found to have abandoned your
permanent resident status. A general guide used is whether you have been
absent from the United States for more than a year. Abandonment may be found
to occur in trips of less than a year where it is believed you did not
intend to make the United States your permanent residence. While brief
trips abroad generally are not problematic, the officer may consider
criteria such as whether your intention was to visit abroad only temporarily
, whether you maintained U.S. family and community ties, maintained U.S
employment, filed U.S. income taxes as a resident, or otherwise established
your intention to return to the United States as your permanent home. Other
factors that may be considered include whether you maintained a U.S. mailing
address, kept U.S. bank accounts and a valid U.S. driver’s license, own
property or run a business in the United States, or any other evidence that
supports the temporary nature of your absence.
What if my trip abroad will last longer than 1 year?
If you plan on being absent from the United States for longer than a year,
it is advisable to first apply for a reentry permit on Form I-131. Obtaining
a reentry permit prior to leaving the United States allows a permanent or
conditional permanent resident to apply for admission into the United States
during the permit’s validity without the need to obtain a returning
resident visa from a U.S. Embassy or Consulate abroad. Please note that it
does not guarantee entry into the United States upon your return as you must
first be determined to be admissible; however, it will assist you in
establishing your intention to permanently reside in the United States. For
more information, see the “Travel Documents” page.
If you remain outside of the United States for more than 2 years, any
reentry permit granted before your departure from the United States will
have expired. In this case, it is advisable to consider applying for a
returning resident visa (SB-1) at the nearest U.S. Embassy or Consulate. An
SB-1 applicant will be required to establish eligibility for an immigrant
visa and will need a medical exam. There is an exception to this process
for the spouse or child of either a member of the U.S. Armed Forces or
civilian employee of the U.S. Government stationed abroad on official orders
. For more information on obtaining a returning resident visa, see the
Department of State’s webpage on returning resident visas.