Fixed-asset investment had been forecast to grow 8.2 percent over the first eight months of the year, which would have marked a slight moderation from 8.3 growth in January-July.
Retail sales rose 10.1 percent in August from a year earlier, the statistics bureau said, cooling from July's 10.4 percent pace and below analysts' expectations for a 10.5 percent increase.
Growth of private investment slowed to 6.4 percent in January-August from 6.9 percent in the first seven months of the year, suggesting small- and medium-sized private firms still face challenges in accessing financing.
Private investment accounts for about 60 percent of overall investment in China.
China is forecasting growth of around 9 percent in fixed asset investment for 2017, and expects retail sales to increase about 10 percent.
The government is targeting annual economic growth of around 6.5 percent this year, down from the 6.7 percent pace clocked in 2016.
Analysts say Beijing should handily meet the 2017 growth target after the economy surprised many by growing at a healthy clip of 6.9 percent in the first half of the year.
However, most China watchers expect activity will slow slightly in coming months as higher financing costs and property market curbs start to weigh on activity.