To systematically analyze the effects of woody biochar application on the growth of ×Taxodiomeria peizhongii seedings in saline-alkali soil, 1.5-year-old ×Taxodiomeria peizhongii seedlings were utilized as experimental material. Four treatment groups were established: control (CK), biochar (B), woody waste (W), and woody waste-biochar mixture (WB). Over a duration of 360 days of continuous field observations were conducted, during which quantitative analyses were performed to assess the differential effects of these treatments on aboveground growth parameters (including ground diameter, plant height, crown width) as well as root growth indices (such as number, total length, surface area, volume) of the seedlings. A multidimensional evaluation system was constructed by integrating the Plant Growth Index (PGI) method and the Biomass Expansion Factor (BEF) method, modified to incorporate root parameters. Experimental results demonstrated that the B group exhibited a 24.86% significant increase in ground diameter growth compared to the CK group (p<0.05). At the same time, plant height and crown width showed growth trends that did not reach statistical significance. The WB group did not display significant growth-promoting effects. Root system development responded more sensitively to biochar than aboveground parts: the B group showed 287.16% and 376.75% significant increases in root surface area and volume, respectively, compared to the CK group, whereas the WB group exhibited 76.01% and 97.19% increases, and the W group showed 54.82% and 58.12% increases. PGI evaluation revealed that the comprehensive growth index of the B group (4.28) was significantly higher than those of WB (2.34), W (2.15), and CK (2.09) (p<0.05). After correction by root volume ratio, the BEF model estimated the biomass per plant in the B group to be 1.05±0.35 kg, representing a 133.05% increase over the CK group (p<0.05), corroborating the PGI assessment. This study confirm the significant biomass accumulation enhancement effect of woody biochar on ×Taxodiomeria peizhongii and innovatively proposes a BEF correction model based on root morphological parameters, providing methodological support for carbon sequestration quantification in forest ecosystems under soil amendment practices.