The secondary xylem of trees serves as the fundamental biological foundation for wood production, and unraveling its regulatory mechanisms has consistently remained a central scientific challenge in basic forest research. In this project, iTraq quantitative proteomics technology was used to analyze proteins related to secondary xylem differentiation in the immature xylem of E. urophylla × E. grandis (E. urograndis) at different ages in response to changes in the age of the trees.