The huge contract comes despite weeks of deteriorating ties over China's detention of a Rio Tinto mining executive and Australia's decision to allow a visit by leading dissident Rebiya Kadeer.
The signing ceremony was held behind closed doors in Beijing and announced by Australia's Resources and Energy Minister Martin Ferguson late on Tuesday.
"The ExxonMobil 50 billion (Australian) dollars LNG export contract to China represents the biggest export contract in the history of Australia," he said, according to public broadcaster ABC.
"It also potentially underpins the biggest single construction project in the history of Australia, which could see 6,000 workers on the job at the peak construction period."
Ferguson said the deal, which follows ExxonMobil's 21 billion US dollar sale of Gorgon LNG to India's Petronet last week, showed Australia had a future as a "global energy superpower".
"This agreement is testimony to the strength of Australia's continuing trade and investment relationship with China," he was quoted as saying."As China continues to develop as a modern global industrial and commercial powerhouse, Australia is committed to walking with it on its remarkable journey."
Finance Minister Lindsay Tanner dismissed recent controversies as mere glitches in a trade relationship worth 58 billion US dollars last year.
"Inevitably there will be the tension here and there, that's unavoidable," Tanner told Sky News on Wednesday. "But underneath that is a developing and very strong commercial relationship that's very important for both countries."
Fast-industrialising China has maintained a big appetite for Australian resources despite the global downturn, pledging up to six billion US dollars in financing for Fortescue Metals as it signed an iron ore deal this week.
China's Yanzhou Coal has launched a 2.8 billion US dollar takeover of Felix Resources, while OZ Minerals sold most of its assets to Chinese firm Minmetals in June.
Chinalco's 19.5 US billion dollar cash injection was turned down by mining giant Rio Tinto, weeks before top executive Stern Hu was detained in Shanghai for alleged spying, later downgraded to industrial espionage.
Relations soured further over a visit by exiled Uighur leader Kadeer, with Australia revealing a "most unhappy" China axed a senior minister's visit in protest.
However, Ferguson said the liquefied gas deal was a "landmark in our relationship with China."
"We are a country built on foreign investment and we continue to welcome investments that develop our resources for the benefit of all Australians," he said.
"PetroChina is an increasingly important partner in the Australian LNG industry and I hope the relationship will be long and successful."
Wang Yong, a professor in international relations at Peking University, said the timing of the deal showed that China was willing to work with Australia to solve disputes.
"China will deal with disputes emerging between the two countries in a cooperative way," he told AFP.