Beijing has tried to diversify its foreign exchange portfolio by creating a sovereign wealth fund that can invest some of the reserves overseas. The government has also encouraged Chinese companies to expand overseas and to acquire mines and natural resources to fuel China’s hungry economy. But because China has too much foreign money for any other outlet to absorb, the vast majority of its fast-growing reserves continue to be destined for the United States bond market.

“China has no choice but to keep buying,” said Zhang Ming, an expert at the Chinese Academy of Social Sciences, a Beijing research group. “After all, U.S. Treasury bonds are still the largest and most liquid investment product in the world.”

All of which has helped enable America’s own fiscally dubious habits.

The United States’ huge deficits — not only in government spending, but in trade and savings as well — have weakened its economy and strangled consumption. Many economists say that would poison the long-term prospects for the dollar, if it were not still the world’s reserve currency and most reliable safe haven.

Helping maintain that role for the dollar are the staggering debt problems that Europe and Japan are struggling with. With global investors like China having few good options besides United States Treasuries, Washington, despite its current debt-ceiling debacle, can continue to hold down interest rates and wallow in cheap borrowing.

Beijing in recent years has frequently fretted aloud about Washington’s monetary policies. In 2009, shortly after the global financial crisis broke out, China’s prime minister, Wen Jiabao, said his country was “worried” about the safety of its huge cache of United States Treasury holdings. Last year Chinese policy advisers criticized the Federal Reserve for undermining the value of holdings by “printing too much money” with its so-called quantitative easing policies.

But even now, despite Beijing’s scolding about the debt impasse in Washington, China’s options may be limited.

”There’s really nothing different they can do,” said Eswar S. Prasad, a Cornell economics professor and former head of the China division at the International Monetary Fund. “Even if China felt the United States was going off a cliff, there’s no other place for them to put their money.”