Age-specific migration rate can be examined from the aspects of level and pattern. After testing the data from different sources and defining the flowing of floating population in China, this paper describes the structure difference between flowing and total floating population, and the trend of time-varying migration age pattern in last 30 years, based on the data of four censuses and the data of two 1% sample censuses since 1990. Results show that: firstly, there are two significant differences of the age structure between flowing and total floating population, that is, the concave of 5-15-year-old children and different age points of inflection after the peak age. Secondly, the age-specific migration rate got improved generally among all age group in last 20 years, with a mode of a sharp increase in peak value (peak shape), a change of inflection points after the peak age, and the delayed age to participate in mobility. So, the flowing of floating population is preferred in the parameter design of population projection in the future. We should understand and solve the problem of floating population in China from the perspective of integrity and difference. A comprehensive, all-age, and all-round policy system is needed so as to ensure the rights and interests of the floating population in an all-round way through systematic institutional arrangements.