China may be one of only two countries on Earth with the manpower to invade the United States (India is the other), but lacks the military forces to conduct such an operation.
First, you have to get control of the Air. As the Battle of Britain showed the world, a naval invasion can not be conducted without Air Supremacy. The U.S. Air Force has fought several wars over the past twenty years, gaining invaluable experience. The PLAAF hasn't fought a real war since Korea in 1953. Further the best aircraft that the Chinese have are Russian Designs that the U.S. has fought and defeated with no air to air losses. Further, the Chinese pilots receive half the flying hours a month compaired to U.S. pilots. This is considered less then needed to be proficient in flying their aircraft, much less then provided the expertise to engage in Air Combat Manuevers. To make matters worse the U.S. has a force of Stealth Bombers that can strike Invasion Staging Areas and Logistical Sites without a chance of being stopped.
Second, while the Chinese have a large navy, it is primary one of destroyers without more then a Division's worth of sealift capability. While the Chinese can in theory mass huge land army, crossing the Pacific in a timely matter is another thing entirely. To land a force Division at a time would invite the destruction of the Chinese Army piecemeal, even if they could get past the Air and Naval problems.
Third, The Chinese haven't fought anyone since Vietnam in 1979. The U.S. Army has been in combat today. Further the bulk of our forces and reserves are in the Continental United States. Any invasion force would be fighting the best we have including M-1 Abrams tanks. The bulk of the Chinese Army uses T-72 or older models (you can't have a large army with the best equipment, there simply isn't enough). T-72s proved to be extremely vulnerable in combat with U.S. Armor.
Fourth, any preperations of an Invasion of the United States would take years in just building up the shipping available. The U.S. could take economic actions that could send the Chinese economy into a tailspin (even with damage to the U.S. economy) as a means of preventing war.
Fifth, if it comes down to it, the U.S. will use Nuclear weapons to protect itself. It would be a last resort, but an invasion would rise to that level. While the Chinese do have it's own ICBMs, we do have some missile defense to counter it, and they have no where near the number of missiles to launch a strike on our silos and bomber bases, and have only a very limited ability to counter of SSBNs.
Six, The Chinese have to maintain a large army on their border with Russia, even if their allies. It's unlikely that any other country will side with China, not wanting to be targeted by U.S. ICBMs.