Chinas economy survives at the mercy of the United States Navy. For thousands of years, China has been what is known as a continental power. That is, it had everything it needed right at home and was not dependent on seaborne trade to survive (like Britain and Japan, which are classic examples of Oceanic Powers.) But now China is an Oceanic Power, with over half of its GDP coming from exports to foreign nations. Moreover, nearly all the oil China uses is imported via seagoing tankers. China is now more dependent on access to the sea than Japan, which  gets about 20 percent of its GPD from exports, or the U.S., which gets about ten percent. Thus if China were to try and take Taiwan by force, the United States could cause economic collapse in China by blockading Chinas ports. This could be done with nuclear submarines, a type of warship China is not equipped to deal with. Then theres the American aircraft carriers, which can clear the sea of any Chinese ships that venture too far from the Chinese coast. While China has some capability to go after American carriers and subs, its not enough to break a blockade. Indeed, the blockade can be established by simply announcing that any ships that violate it will be seized, or sunk. This is because American satellite surveillance can track ships movements accurately. China can threaten nuclear retaliation, but even there they are at a major disadvantage, and to make that threat, opens them to a first (non-nuclear or nuclear) strike against their ICBMs (which at present can only reach part of the west coast of North America.) 

While Chinas military power is growing, it will be decades before they become strong enough to change the above situation. So any serious threat to Taiwan has to be made under the threat of major economic retaliation. While such a blockade would initially give the Chinese government a boost in popularity among Chinese. A few weeks or months of several hundred million Chinese being jobless would change attitudes, given that the current communist dictatorship is not very popular to begin with.