Abstract: Three-dimensional mathematics model was established by analyzing generation, decay and diffusional transport law of thoron and it’s daughters in dwellings. The spatial distribution of thoron and it’s daughters in dwellings were researched by finite element calculation. Though average concentration of thoron and it’s daughters has a little change with increase of the numbers of finite element, the more the number of finite element is, the more exact it is. The equilibrium factor of thoron and it’s daughters was calculated according to three-dimensional distribution in dwellings, which may be reasonable.