To improve the performance of BP decoding algorithm of digital fountain codes on wireless channels, a delta decoding algorithm based on the ripple set is proposed. The algorithm analyzes the likelihood ratio threshold of the variable nodes. When the likelihood ratio of the variable node is greater than the threshold, it can be successfully decoded in advance. On the other hand, when the overhead is increased, we can delete those variable nodes that have been decoded, and decode the nodes that have not achieved the decoding threshold, to further reduce the amount of calculation. The simulation shows that the performance of the new algorithm is as good as the traditional BP decoding algorithm, but the decoding efficiency is greatly improved.