Revealing the differences in foliar traits and leaf spectral characteristics between new and old leaves is important for the non-destructive monitoring of vegetation physiological and ecological parameters and can provide more theoretical support for quantitative remote sensing in forestry. This study sampled new and old leaves of 8 tree species in Changbai Mountain, and their reflectance spectra were measured. Then, the first-order derivative transformations and spectral indices were calculated. Foliar traits such as specific leaf area (SLA), leaf water content, leaf nitrogen content, and leaf carbon content were measured in our laboratory. Using variance analysis and correlation analysis, the differences in physicochemical properties and spectral characteristics between the old and new leaves of different tree species were investigated, and the differences in related coefficients were also analyzed between the old and new leaves. The results show that: (1) Multiple foliar traitsof thetree species showed significant differences between old and new leaves. Except for leaf carbon content, which did not differ significantly between old and new leaves, the other three traits showed significant variability between old and new leaves. (2) The differences in the spectral characteristics of different tree species were inconsistent between the old and new leaves. Only the old and new leaves of Betula costata, Ulmus laciniata, Acer buergerianum, and Pinus koraiensis showed more obvious differences in spectral curve characteristics. Betula costata showed significant differences in the spectral trilateral characteristics. (3) The correlation between leaf traits and spectra showed significant differences between old and new leaves, and the spectra have different abilities to indicate leaf traits. Near-infrared spectrum spectral reflectance is a better indicator of leaf nitrogen content for old leaves than for new leaves. In contrast, many spectral indices indicate better water and leaf carbon content in newer and older leaves. This study shows that there are not only differences in leaf properties and spectral characteristics but also differences in their correlations between old and new leaves. This study has a guiding significance for selecting representative leaves in the non-destructive observation of forest leaf properties.