The externality of education means that the holding and output of education will not only achieve the educational goals of the state and individuals, but also bring economic and non-economic influences to other social groups and individuals, but education cannot give direct compensation or unnecessary compensation to the affected social groups and individuals. The externality of education is positive, diverse, lasting and difficult to quantify. The definition, classification and summary of educational externalities can better provide theoretical basis for solving the problem of educational externalities.