Why is china the most populated country ?

With more than 1.3 billion people (1,336,718,015 as of mid-2011), China is the world's most populous country. As the world's population is approximately 6.7 billion, China represents a full 20% of the world's population so one in every five people on the planet is a resident of China.

China's population growth has been somewhat slowed by the one child policy, in effect since 1979.

As recently as 1950, China's population was a mere 563 million. The population grew dramatically through the following decades to one billion in the early 1980s.

China's total fertility rate is 1.7, which means that, on average, each woman gives birth to 1.7 children throughout her life. The necessary total fertility rate for a stable population is 2.1; nonetheless, China's population is expected to grow over the next few decades. This can be attributed to immigration and a decrease in infant mortality and a decrease in death rate as national health improves. 

By the late 2010s, China's population is expected to reach 1.4 billion. Around 2030, China's population is anticipated to peak and then slowly start dropping. 

In the next few decades, India, the world's second most populous country is expected to surpass China in population. By 2040, India's population is expected to be 1.52 billion; that same year, China's will be 1.45 billion and India will become the world's most populous country. As of 2005, India has a total fertility rate of 2.8, well above replacement value, so it is growing much more quickly than China.