MMicroRNAs (miRNA) are a class of non-coding small RNAs that function as regulators of development and stress responses in plants. The ptc-MIR171 gene family is an old gene family which consists of 14 members. In this paper, the duplication, expression patterns, promoters and target genes of poplar MIR171 gene family are analyzed. The results showed that poplar MIR171 gene family expanded mainly through large-scale duplication 48-54 million years ago (MYA) and the expression patterns and functions have diversified. Poplar MIR171 gene family involves in complex regulation of development, light responsiveness and photomorphogenesis by targeting GRAS transcription factors and signal transduction proteins.