所謂替代遷移是指遷移人口對長期處於更替生育水平以下的人口，因人口自然減少而產生的替代效用。北京、上海、天津已經完成了人口轉變，實現了低生育水平，遷移人口和人口年齡結構變化將對人口未來發展趨勢起主導作用。本文採用替代遷移研究方法，實證分析了三大城市在無遷移人口、保持峰，值人口、維持峰值勞動年齡人口和潛在供養比最大值四種條件下的遷移人口的替代規模和替代效應，認為適度替代遷移人口是解決長期低生育水平人口問題，的有效途徑。

Beijing, Shanghai and Tianjin are experiencing below replacement fertility. Under such fertility trends, these cities will eventually face negative population growth rates. This paper applies the concept of "replacement ,migration" to examine how many migrants will be needed if these cities are to ,maintain their peak population size under a no migration situation, their peak ,working labor force, or their peak ratio of population aged 15-65 to that of 65 and over. The results seem to indicate that the migration volume under the scenarios of maintaining peak population size and peak working population size could provide reasonable and acceptable consequences. Furthermore, for cities ,experiencing below replacement fertility levels, the migration volume needed to maintain either peak population size or peak working population size varies positively with the size of the below replacement deficit.