The HSBC Flash China Manufacturing Purchasing Managers Index dropped from 51.8 in April to 51.1 in May, Markit said.
A flash estimate is based on data collected from 85 percent to 90 percent of results of a survey.
"Manufacturers continue to reduce inventories amidst slowing new business flows, leading to slower production growth at a 10-month low," said Hongbin Qu, HSBC chief economist.
"That said, we think there is no need to worry about a hard landing because the current level of the PMI is still consistent with around ... 9 percent gross domestic product growth," the economist said.
"Policy focus is still tilted toward taming inflation. We expect Beijing's tightening policy will continue in the coming months," he said.