The rural fundamental and productive fixed-asset investment not only makes active influence on the changes of farmers’ operational, wages and property income, but it also has an optimal scale range for farmers’ income increase. From the perspective of farmers’ income increase, this article evaluates the optimal scale of rural fixed-asset investment by setting up model with statistic data, and the results show that the optimal scale of per capita rural fixed-asset investment is 76.35% of per capita net income of rural residents, which has been reached in China in 2009. Therefore, compared with the adding of rural fixed-asset investment, a better income increase effect can be achieved through the adjustment of rural fixed-asset investment structure.