[en] To investigate the level of thoron (²²⁰Rn) concentration and its characteristics, the concentration of radon (²²²Rn) and ²²⁰Rn outdoor were measured using radon monitor named RAD7 in the surrounding region of a uranium mine. The main factors affecting the concentration of 220 Rn and its progeny at outdoor were discussed, and the annual effective dose from ²²²Rn and ²²⁰Rn to the residents living around the uranium mine was estimated too. The findings show that the ²²⁰Rn concentration in the survey areas is at natural background level, its frequency distribution type shows skewness, and ²²⁰Rn concentration is greater than ²²²Rn concentration at nearly eighty per cent monitoring sites. The concentration of ²²⁰Rn and its progeny at outdoor is mainly affected by the specific activity of the natural radionuclide ²³²Th in soil and the seasonal factors. The annual effective dose from ²²⁰Rn and its progeny accounts for about 20% of that from ²²²Rn, ²²⁰Rn and their progenies in the survey areas, and so effective dose from ²²⁰Rn can not be ignored. (authors)