The preliminary reading of Caixin's Purchasing Manager's Index (PMI) came in at 47.1 this month, the Chinese media group said in a joint statement with Markit, a financial information services provider that compiled the survey.
The figure, which fell from July's final reading of 47.8, was the worst since a reading of 44.8 in March 2009, according to Markit's data.
It was also significantly weaker than the median estimate of 48.2 from a poll of economists by Bloomberg News.
Caixin took over sponsorship of the PMI survey from British banking giant HSBC last month.
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"There is still pressure on the front of maintaining growth rates," He Fan, an economist at Caixin Insight Group, said in the statement.
"To realise the goal set for this year, the government needs to fine tune fiscal and monetary policies to ensure macroeconomic stability and speed up the structural reform," added He.
Beijing earlier this year set the annual target for economic growth at "around seven percent".
Nomura economists said Friday's PMI data suggested growth momentum had weakened in the July-September period.
"We expect monetary policy easing to continue," they said in a note.
Authorities accept the need to steer China's growth lower to make it more sustainable and driven by consumer demand rather than investment, but have taken stimulatory measures to put a floor under the slowdown.
Chinese stocks -- which have been extremely volatile in recent months -- extended falls after the figures came out, with the benchmark Shanghai Composite Index down 3.04 percent by the break.
Factory closures in Beijing and surrounding areas to ensure blue skies above the notoriously polluted city for a huge military parade next month commemorating victory over Japan in World War II were another factor, he said in a report.
"We still think the downside risks to short-run growth are now overstated," he said, adding the government "has plenty of policy ammunition" and will not allow growth to slip much further.