More than 221m Chinese were online at the end of February compared with 137m at the start of 2007, putting it level with the US, according to statistics from the China Internet Network Information Centre.
However experts say that the number is sure to have risen steeply in the past few weeks putting China in the number one spot.
Despite the increase, internet penetration in China remains low given the size of the population, with only 16% of the country's 1.3bn people online compared with a world average of 19%.
The number is expected to rise rapidly in the next few years as hundreds of millions of Chinese experience a rise in their incomes.
BDA China, a Beijing technology company, said the Chinese online population should keep growing by 18% annually, reaching 490m by 2012, a number larger than the entire US population.
Sites that offer video sharing have become popular in China over the past year and attract up to 100m visitors a day.
The government, which blocks searches for content considered subversive or pornographic, has now started focusing on these sites.
Last month, the government said it would shut down 25 Chinese video sites and punish 32 others for violating new rules against carrying content that is deemed pornographic, violent or a threat to national security.
The most commonly blocked searches are for words such as Taiwan independence, Tibet and the Dalai Lama.