At present, the capability to increase income by farmers themselves in China is weak and the income increase is unstable. The root reason for it is that low level of employability restricted by low level of human capital investment limits income increase. According to the investigation data, the analysis shows that farmers’ income increase comes from the enhancement of employability, especially the promotion of position competence which is restricted by human capital investment, especially education level. Farmers’ income level influences the human capital investment level. Employability has fully mediating effect on human capital investment influencing non-farming income for farmers, and has partly mediating effect on human capital investment influencing farming income. “Education-position competence-income” is the core transmission mechanism in human capital investment influencing farmers’ income. Therefore, the essential way to stabilize farmers’ income is to enhance farmers’ employability by promoting human capital investment level.