Two-stages interpolation is developed to improve the computational efficiency of trajectories of water droplets simulation based on the Lagrangian method. The first stage is collection efficiency interpolation. In this stage, the collection efficiency on all wall grid is interpolated using radial basis functions from the collection efficiency of the water droplets trajectories which strike on the wall. The second stage is trajectories interpolation. The movement of the droplets is simulated by two stages. Only a little water drops trajectories are used in the far field. Plenty of water droplets trajectories are employed near the airfoil. For the first-stage interpolation, linear interpolation or high order spline interpolation underperform under the condition of ice accumulation, especially for the blocking area. To overcome this problem, the radial basis functions interpolation method has been introduced into the collection efficiency interpolation. The results show that the computational efficiency has been improved greatly and the radial basis functions interpolation avoids miscalculation in blocking area.