Abstract: A mathematical model and Monte-Carlo simulation method of radon exhalation from uranium tailings were developed based on the nuclear recoil theory. The grain size effect on radon exhalation under different moisture contents was simulated. When radium is distributed on grain surface, K increases with grain size and then remains a constant value under moisture contents of 5%, 20%, 50% and 75%. When radium is distributed to a depth of 34 nm or 68 nm from the surface, K firstly increases with grain size and then reaches a constant value under moisture contents of 5% and 20%, and K decreases with the increase of grain size and then becomes a constant value under moisture contents of 50% and 75%. The radon exhalation rate of the uranium tailings samples was calculated by Monte-Carlo method. The calculated radon exhalation rate agrees well with that obtained by experiment, and the relative error is 3%-9%.