In this study, the boosted regression trees (BRT) method was used to analyze the main factors that influenced the above-and underground biomass conversion and expansion factor (BCEF) of Masson pine. Data from the permanent sample plot of the 8^th Chinese National Forest Inventory for Fujian Province was used. The results showed that the cumulative relative influences of stand characteristic and topographic factors on above-and underground BCEF were 87.20 and 86.59%, respectively. Stand characteristic and topographic factors were the most important factors for both above-and underground BCEF. Among the stand characteristic factors, the age group was dominant, contributing to 41.13% of the aboveground BCEF differences that decreased with an increase in the age group, and 41.54% of the underground BCEF differences that increased with an increase in the age group. Among the topographic factors, slope was the key variable, contributing to 14.52% of aboveground BCEF differences, which were the highest at the southeast slope and the lowest at the west slope, and 15.16% of the underground BCEF differences, which were the lowest at the southeast slope and the highest at the west slope. In addition, soil factors had a lower effect on both above and underground BCEF. The relative influences of soil factors on above-and underground BCEF were 12.80 and 13.41%, respectively. Among the soil factors, the thickness of the humus layer was the key variable, contributing to 9.02% of the aboveground BCEF differences and 9.13% of the underground BCEF differences. Among all influencing factors, age group had the highest effect on both above-and underground BCEF. Consequently, the ways, such as calculating BCEF based on age group or building universal BCEF models of all age, could achieve a better precision of biomass.