On the basis of Mie scattering theory, simulation of circularly polarized laser beam propagating in horizontal atmosphere is designed using Monte Carlo method with the consideration of polarization character. The simulations reveal that the polarization performance at the receiving spot center is fine and better than at the edges as atmospheric visibility is 5 km and transmission distance is 11.16 km. The depolarization characteristics of left-handed circularly polarized laser beam propagating in horizontal atmosphere at the distance of 11.16 km were measured. As a result, under the conditions of atmospheric visibility is 5-10 km and C_n² is 1.825×10^-13 m^-2/3, the rotation of left-handed circularly polarized laser beam doesn′t change and the polarization performance at the receiving spot center is better than at the edges. The experiment result has the same trend as simulation result.