The credit rating agency says the Fed plans announced Thursday to buy mortgage bonds will likely hurt the economy more than help it.
Egan-Jones says the plan will reduce the value of the dollar and raise the price of oil and other commodities, hurting businesses and consumers.
It is the second downgrade of the U.S. by the company.
In April, Egan-Jones downgraded the U.S. to AA from AA+. The company stripped the U.S. of a top AAA rating in July 2011.
Egan-Jones is a much smaller but well-known competitor to the big three rating agencies: Moody's Investors Service, Fitch Ratings and Standard & Poor's.