The Caixin General China Manufacturing Purchasing Managers' Index (PMI), an indicator of factory activity based on a private survey, fell to a three-month low of 49.2 in May.
The reading, released on Wednesday after research by financial information service provider Markit sponsored by Caixin Media, was down from 49.4 in April and 49.7 in March, signalling a marginal deterioration in the manufacturing sector.
A reading above 50 indicates expansion, while a reading below 50 represents contraction.
The data came on the heels of the official PMI that showed manufacturing activity stayed in expansionary mode for a third consecutive month in May.
The official PMI came in at 50.1 last month, unchanged from April, according to data released by the National Bureau of Statistics and the China Federation of Logistics and Purchasing.
The official PMI samples 3,000 relatively large enterprises in China. The Caixin PMI samples 420 small and medium-sized manufacturing enterprises and is relatively volatile due to its small sample size and the dominance of small enterprises.