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移民进入美国时护照要有起码60天的有效期。所以如果你想移民签证有6个月的有效期的话，护照就要要有6个月+60天的有效期。但是如果你的护照有效期更短，他们可以把移民签证有效期剪短，让它在护照过期前60天过期，那样就可以在旧护照上签发移民签证。看9 FAM 504.10-2(A)(2)：

(2)  (U) Limitation of Visa Validity to Meet Passport Validity Requirement:  If an immigrant is required to present a valid passport, determine whether the applicant’s passport is valid for at least 60 days beyond the validity of the visa, as required in 22 CFR 42.64(b).  You may limit the validity of the visa to less than six months if the passport is valid for 60 days plus a period sufficient to enable the applicant travel to the United States.

没有要求移民入境护照要有6个月的有效期；那个只是针对非移民的。