Tree mortality correlates with many biotic and abiotic variables. However, less is known about how the relationship between influence factors and tree mortality changes with tree size. To understand size-dependent relationship between drivers and tree mortality, we investigated tree mortality of Populus simonii Carr over a wide range of tree sizes in 100 plots in Bashang Area. We divided trees into three size classes based on the magnitude of the effects of influence factors on tree mortality. The random forest (RF) was used to find out the key factors for each size class. The results showed that the trees were categorized into small trees (DBH < 10cm), medium trees (10 ≤ DBH < 25cm), and large trees (DBH ≥ 25cm). The relative basal area (RBA) was the most important predictor for mortality of small-sized trees. The tree mortality rate was the lowest with 0.23 < RBA < 0.43. Density was the most significant influence factor for mortality of medium-sized trees. When stand density was higher than 600 stems hm^-2, the tree mortality rate increased dramatically with the growth of stand density. The soil moisture content of 20-60 cm and stand basal area (SBA) were the most important two predictors for mortality of large trees. The tree mortality rate declined sharply when soil moisture content of 20-60 cm reached 5%. Biotic and abiotic variables could explain 70%-90% variation in tree mortality with RF model. Thus RF model can be used in the tree mortality prediction and provide important information for the local forest management and plan.