A key private index of China's manufacturing sector has reached its highest level in more than one-and-a-half years.

The HSBC Purchasing Managers' Index rose to 51.5 in December, up from 50.5 in November, indicating modest but accelerating growth in China's manufacturing sector.

An index reading above 50 points indicates that conditions in the manufacturing sector are improving - the further above 50, the greater the improvement.

The reading of 51.5 was the highest since May 2011, and the index also recorded the fastest growth in Chinese manufacturing output in 21 months.

In a positive sign for future activity, the index of new orders reached its highest level in almost two years.

HSBC's China chief economist Hongbin Qu says such figures indicate that the Chinese economy may grow around 8.6 per cent next year.

"December's final manufacturing PMI picked up for the fourth consecutive month to a 19-month high, thanks to the faster new business flows and the end of destocking," he observed.

"Such a momentum is likely to be sustained in the coming months when infrastructure construction runs into full speed and property market conditions stabilise."