Channel coding is the key technology to solve the atmospheric turbulent channel influence, which can overcome the weakening on the performance of free space optical communication caused by atmospheric attenuation and light intensity fluctuation. Digital fountain code is a channel coding method without fixed rate, based on the distribution design. This paper presents the method of switch degree distribution and gives the optimal value of the switching point. The simulation result shows that regardless the turbulent environment of free space optical communication system is weak or strong, digital fountain code compared with RS code has the better coding gain. Also, under the same SNR, free space optical communication system with the lowest bit error rate and the least information rate decline is under digital fountain code of switch distribution compared to binary exponential distribution and robust soliton distribution. At the same time, the double switch degree distribution compared with the single degree distribution improves more on the performance of free space optical communication system. Therefore, digital fountain code of the double switch distribution can effectively suppress the influence of atmospheric turbulence, significantly improve the performance of free space optical communication system.