The decision technically means the world's biggest economy is less able to pay its $US14.3 trillion ($A13.7 trillion) debt, and is now deemed a worse credit risk than Australia, Canada and France.
America's bankers are concerned about its spending problem.
"China, the largest creditor of the world's sole superpower, has every right now to demand the United States address its structural debt problems and ensure the safety of China's dollar assets," the Xinhua commentary said.
It urged the United States to cut military and social welfare expenditure.
S&P cut the long-term U.S. credit rating by one notch to AA-plus on concerns about the government's budget deficits and rising debt burden. The move is likely to raise borrowing costs eventually for the American government, companies and consumers.
"The downgrade reflects our opinion that the fiscal consolidation plan that Congress and the Administration recently agreed to falls short of what, in our view, would be necessary to stabilize the government's medium-term debt dynamics," S&P said in a statement.
