A method for estimating the total inputs of organic matter to farmland was proposed in the paper. A typical gully area of loess plateau region, Changwu County was selected as the study area. The organic matter input to farmland can be divided into four parts: the crop straw returned, the crop straw residues, the underground biomass from crop’s root, and the organic fertilizer applied. Based on the LANDSAT 5 satellite remote sensing data, the net primary productivity (NPP) based on CASA model was estimated, and then the total biomass of crop was acquired. Combined with the crop root/shoot ratio, the weights of crop biomass above ground and underground biomass were estimated. The weight of straw residues and straw returned were estimated by biomass above ground and the date obtained through field survey. The results showed that the total organic matter input to farmland was 42 700 t per year in the study area.