Referring to the relevant researches on international migrants,this essay describes the rural- urban migrants' cultural acculturation by comparing the old generation and the new generation,and studies the relation between acculturation and mental health among them by analyzing dimensions of cultural acculturation,including attitude,behavior and psychology. It finds out that there is a generational difference in the rural- urban migrants' cultural acculturation. The cultural acculturation of new generation is characterized by three low and one high. Moreover,the way that cultural acculturation influences rural- urban migrants' mental health is also different between two generations. It has an implicit effect on new generation's mental health by affecting their thoughts and attitude,but has an explicit effect on old generations by affecting their behaviors and life style.