HSBC Corp.'s purchasing managers index fell to 50.2 from July's 18-month high of 51.7 on a 100-point scale on which numbers above 50 show an expansion. An official industry group, the China Federation of Logistics and Purchasing said its separate PMI declined to 51.1 from 51.7.
"We think the economy still faces considerable downside risks to growth in the second half of the year, which warrant further policy easing to ensure a steady growth recovery," said HSBC economist Hongbin Qu in a report.
China's economic growth edged up to 7.5 percent in the three months ended June 30 from the previous quarter's 7.4 percent. But that was supported by higher government spending on building railways and other public works.
Analysts say without further government support, growth is likely to decline.
The latest surveys showed weaker demand in exporter orders, investment and hiring.
"The August PMI was significantly reduced, showing there is downward pressure in current economic activity," said Zhang Liqun, a government economist quoted in the Chinese federation's report.