The Ministry of Commerce said the January figure was worse than December's year-on-year decline of 4.5 percent.
The ministry said FDI inflow has been going down since June of last year and blamed the global economy and rising labor costs at home.
Xinhua News Agency reported China's manufacturing sector got the largest share of the January total, accounting for $4.34 billion, but that was down 5.8 percent year-on-year.
The service sector, which accounted for $4.03 billion in January, saw a bigger decline of 9.8 percent from the same month of last year.
FDI into China for all of 2012 was down 3.7 percent year-on-year to $111.72 billion.
The commerce ministry has said FDI into China this year will stay at the level of 2012 as domestic consumption and investment are likely to maintain steady growth.