Abstract: Based on the Forest Ecosystem Services Assessment Standards (LY/T 1721－2008) issued by the State Forestry Administration, this paper evaluated four key functions of forest ecosystems, i.e., water conservation, soil conservation, carbon fixation and oxygen release, and nutrient accumulation. Focusing on the project area of Majiang County in Guizhou Province, this study provided some quantitative evidence that the implementation of the small hydropower substituting fuel project had positive effects on the values and material quantities of ecosystem service functions. The results showed that the small hydropower substituting fuel project had a significant effect on the increase of forest ecosystem services. Water conservation quantity of Pinus massoniana and Cupressus funebris plantations inside project area was 20662.04 m³·hm^-2·a^-1, 20.5% higher than outside project area, with soil conservation quantity of 119.1 t·hm^-2·a^-1, 29.7% higher than outside project area, carbon fixation and oxygen release of 220.49 t·hm^-2·a^-1, 40.2% higher than outside project area, and forest tree nutrition accumulation of 3.49 t·hm^-2·a^-1, 48.5% higher than outside project area. Small hydropower substituting fuel project for increasing the quota of forest ecosystem service function value was in the order of carbon fixation and oxygen release function (71400 yuan·hm^-2·a^-1) > water conservation function (60100 yuan·hm^-2·a^-1) > tree nutrition accumulation function (13800 yuan·hm^-2·a^-1) > soil conservation function (8100 yuan·hm^-2·a^-1). Small hydropower substituting fuel project played an important role for improving the forest ecological service function value and realizing the sustainable development of forest.