Background: The radon migration research result shows that the migration of Rn is directional. Purpose: We aim to research further the concentration distribution of accumulated Rn in the horizontal direction and within the confined space in a long time. Methods: A measuring device was designed based on the CR-39 type solid track detector to measure Rn concentration distribution in the horizontal direction. The method of least squares fitting was applied to fit anti-diffusion coefficient that affects the distribution of Rn and its daughters within different horizontal distances. Results: The anti-diffusion coefficient and the measured distance in horizontal direction were related with a negative power function

with the increase of horizontal distance

the anti-proliferation effect on Rn concentration distribution were gradually weakened. Conclusion: With the Rn concentration of 45 Bq·m-3

the anti-diffusion coefficient decreases sharply within the distance of 0-4 cm and then tends to be steady.