China's economy will have shown improvement this year but slow slightly in 2018, a top state think tank forecast on Wednesday, with government investment seen easing as leaders look to recalibrate the country's growth dynamic.

But while the Chinese Academy of Social Sciences (CASS) estimate was better than its previous outlook it warned of possible trade headwinds from US President Donald Trump's protectionist leanings.

At a briefing to release it its annual outlook, CASS said the world's number two economy would expand 6.8 percent in 2017 and tick downwards to 6.7 percent next year.

The forecasts come as Chinese leaders wrap up a key economic meeting known as the Central Economic Work Conference taking place behind closed doors in Beijing.