Multiple types of channels may be involved in UAV communication scenarios, which can impact communication links, causing link disconnection and high bit error rates. Fountain codes can effectively relieve the above impacts. They not only improve the reliability and stability of the link but also provide a reliable end-to-end communication solution. Numerous studies have shown that optimizing the degree distribution can effectively improve the performance of fountain codes. Employing combined degree distribution in fountain code design has proved to be an effective way to further enhance their performance. This paper proposed a novel combined degree distribution design method and a proportional coefficients optimization method. The design and optimization method can improve the performance of fountain codes by increasing the amount of member degree distributions in the combined degree distribution and optimizing the proportional coefficients of the members taking the minimum decoding overhead under a specific symbol error rate as the objective function. The simulation results show that the combined degree distribution designed and optimized using the methods proposed in this paper can effectively improve the performance of fountain codes.