The official Purchasing Managers' Index, or PMI, which is not affected by the working-day distortions, fell to 50.6 in April from 50.9 in March despite favorable seasonal factors.

"We expect it to drop below 50 in May," said Zhang.

For Zhang, there are two key risks to 2013 forecasts. The larger risk is policy uncertainty, as political pressure could force the government to maintain its currently loose policy stance longer than expected. The other is external demand, given the uncertain outlook for the European Union and the United States.

According to the Bank of America Merrill Lynch Fund Manager Survey for May, a quarter of the respondents said a hard landing in China and a commodity collapse are their number one "tail risk", an increase from 18 percent in April. About 8 percent of fund managers in Japan, the Asia-Pacific rim and global emerging markets expect China's economy to weaken over the next 12 months, compared with about 9 percent who said a month ago it would strengthen.

Meanwhile, a research note from Spain's BBVA said that despite the weaker-than-expected performance, China's government still has room to adjust its monetary and fiscal policies if the economy continues to weaken, as inflation has been well under target.

China's Consumer Price Index, a main gauge of inflation, rose 2.4 percent in April from a year earlier.