Expansin is an indispensable cell wall protein, involved in plant growth and resistance to stress. Based on the sequencing genome data, a total of 34 expansin genes were identified on 14 chromosomes in Salix purpurea, which were grouped into 4 subfamilies. The analysis of the physicochemical properties showed that most expansin proteins were in weakly alkaline, had a higher aliphatic index and more stable protein structure. The gene sequence was more conservative among the members in the same subfamily with the amino acid identity between 34.2% and 99.4%, while that was between 15.0% and 39.8% among the members in the different subfamilies. Also, the same subfamily genes had similar exon-intron structures, but the different subfamily genes showed that much differently. The codon usage analysis revealed that there are 5 high-frequency codons in Salix purpurea expansin genes. The codon usage bias was much affected by natural selection. The synteny analysis showed that the expansin genes divided in corresponce to whole-genome duplication events in willow. The results here would help us to understand well the role and status of expansin genes in the growth and development of Salix purpurea. It will be helpful for further study on the expansins functions.