The integration of rural and urban in infrastructural facilities and some public services has obtained remarkable achievement, but in area of education, the gap between rural, town and urban educational attainment did not appear a trend like Kuznets curve with general development of education. Analyzing historically, the growth rate of educational attainment in town and rural is lower than in urban. By theoretical analysis and empirical test to key factors and mechanism which would affect educational attainment, the study found that the social foundation for producing and broadening gap is cultural reproduction through dominant and recessive channels in urban and rural respectively; increased investment in high school promote the educational attainment in urban mostly, differences on education finance between urban and rural junior school result in broaden educational attainment gap directly, meanwhile the effects in primary school is not significant statistically; migration of nonnative residents cause the heterostructure of population in urban and rural and then lead to broadening of gap; immigration of registered residents promote educational attainment in urban as well as in town, and emigration lead to broaden urbantown gap even after offsetting the effect of immigration.