For the effective use of wind resources, and in order to decrease the wake effect on the performance of wind turbine, a numerical simulation has carried out to the aerodynamic interaction between two rotors coaxial arrangement. At last the numerical simulation results are verified by experiment. The results show that downstream turbine makes the upstream diverging wake to be converged when coaxial arrangement, and the output of downstream turbine is affected by upstream wake when the distance is less than 5D.The wake field become more diffused after pass the downstream turbine, the area of diverging wake become larger than the wake of upstream turbine mainly due to the increase of turbulence intensity. The greater fatigue load will impact on the downstream turbine. At last the simulation results are in well agreement with the experimental results.