The Russian-designed No 1 generator of the Tianwan nuclear power station, which has an installed capacity of 1,060 megawatts, would begin commercial production by the end of this year, the official, who declined to be named, told Reuters.
Officials began loading fuel to the generator last week and it proceeded smoothly, he said.
The first phase of the Tianwan nuclear station has two 1,060-MW generators, the largest Sino-Russian co-operation project. The official said a start-up date for the second generator had not yet been confirmed.
Beijing has not revealed any information on the second phase of the Tianwan project.
A senior Russian official has said China might be considering ordering six more of its generating units.
Global competition for China's growing nuclear market has been fierce in recent years, with France, the United States and Russia vying to win lucrative contracts to expand China's energy production.
The official Xinhua news agency said the nuclear site in Tianwan could accommodate eight generators each with at least 1,000 MW of generating capacity.
Upon completion, they could generate a total 60-70 billion kilowatt hours of electricity a year, it said.
Energy-thirsty China plans to invest some 400 billion yuan ($48.33 billion) in building around 30 new nuclear reactors by 2020.
It currently has nine working reactors generating around 2.3 percent of its electricity, but aims to boost the proportion of nuclear power to 4 percent within 15 years.