The study of radon release and migration is the basic theoretical issue for rado n measurement, which has a close relationship with extensive applications in geo science, environmental science and so on. In this paper, a new theory of migrati on of radon and its daughters has been discussed. The mechanism of vertical mig ration of radon and its daughters is: after α-particles emitted by radon and it s daughters decelerate, they become He nucleus. The He nuclei can combine with r adon and its daughters and form clusters. Because He is very light, it can make the specific gravity of the clusters lower than their buoyancy in the medium. As a result, the clusters will rise of their own accord. When the specific gravity of the clusters is higher than their buoyancy, the clusters will drop down. The theoretical distribution equation is established to describe radon migration an d proved by experiment as well.