The overall characteristics of college students' employment difficulties are "employment", which leads to the unemployed, the optimal human capital unemployment and the underemployment of college graduates. The root cause of college students' employment difficulties is the depreciation of human capital stock, that is, the ability of human capital carriers to gain income, and weaken or disappear the capacity of income space. The essence is that the expected return on investment can not be obtained. The fact that university graduates are unemployed in our country belongs to the phenomenon of depreciation of human capital stock. It is concluded that the causes of the underprivileged groups include initial human capital investment decision-making mistakes, employer's avoidance of human capital investment, oversupply of human capital market, economic situation, the overall demand is shrinking, and the salary level needs to be revised.