The invention provides an extensin gene of populus deltoids as well as coded protein and application thereof. A nucleotide sequence of the gene is as shown by SEQ ID NO: 1. An amino acid sequence of the coded protein of the extensin gene is as shown by SEQ ID NO: 2. According to the extensin gene of the populus deltoids as well as the coded protein and the application thereof, a fusion expression vector is constructed by utilizing a PdEXT gene and a vector containing a plant fibrovascular tissue specific promoter proNAC068; a transgenic plant is obtained. Shown by a test aiming at the quality character of lumber of the populus deltoids, the wood property of a tree can be effectively improved through the trangenosis of the gene.