Calculating the investment benefits of soil and water conservation(SWC)is an important part in SWC project evaluation.Using the existing methods of benefit calculation,however,is difficult in eliminating the contributions of other factors except SWC and estimating the comprehensive benefits of system.By taking Changting County for an example,an analysis on the contributions of SWC investment to local agricultural economy was made based on the Cobb-Douglas production function theory.Results reveal that the contribution of agricultural investment was highest in the features of output,butlower in technology and system innovation.From 1999to 2009,5.20% of agricul tural GDP were derived from SWC investment and the ratio of output to input was 5.9in Changting County.With the rapid growth of physical capital investment in agriculture,the contributions of SWC investment to agricultural GDP were being reduced year after year,which may restrain farmers'investment enthusiasm for SWC.