【Objective】 Using tree-level variables and single-tree competition indicators, a nonlinear mixed-effects model was used to construct a single-tree leaf area model of Larix olgensis-Fraxinus mandshurica, providing a theoretical basis for further research on stand productivity and canopy structure.【Method】 A total of 111 Larix olgensis plants and 113 Fraxinus mandshurica plants were selected from different mixing proportions of Larix olgensis-Fraxinus mandshurica mixed forest in Shangzhi City, Heilongjiang Province, and their leaf areas were measured. The whole subset regression method was used to establish a nonlinear single-tree leaf area prediction model for the two tree species. The contribution of each variable to the model was analyzed through the relative weight method. Simultaneously, the random influence of the sample plot on leaf area was considered, a mixed effect model was constructed, and the model was evaluated.【Result】 The optimal mixed effect model of larch single-tree leaf area considering random effects at the plot level was composed of P_CR (crown ratio), D_BH (diameter at breast height, DBH), and P_HDH (ratio of forest tree height to the average height of dominant trees in the forest stand), including one random effect parameter. The R_{\mathrm{a}\mathrm{d}\mathrm{j}}^2 of the model was 0.89, root mean square error (RMSE) was 11.68 m², mean deviation (ME) was -0.202 7 m², mean absolute deviation (MAE) was 7.943 0 m², and prediction accuracy (P_a) was 99%. The optimal mixed effect model of ash single-tree leaf area considering the random effect at the plot level consists of P_CR, D_BH, P_HDH, and P_CW (crown width), including one random effect parameter, The R_{\mathrm{a}\mathrm{d}\mathrm{j}}^2 of the model was 0.87, RMSE was 13.61 m², ME was -0.281 7 m², MAE was 9.397 6 m², and P_a was 99%, all of which had good fitting effects. The relative weight calculation results showed that in the optimal model, D_BH was the variable that has the greatest impact on the single-tree leaf area of Larix olgensis and Fraxinus mandshurica. 【Conclusion】 The mixed effect model considering the plot level improves the accuracy of predicting the leaf area of single trees of the two tree species. DBH is the variable that has the greatest impact on the leaf area of a single tree. It is necessary to consider tree competition variables in leaf area models of mixed forests. The model constructed in this study can provide technical support for accurately predicting the leaf area of single trees of Larix olgensis-Fraxinus mandshurica and can help with conducting in-depth research on the growth, development, and crown structure of the forest stand.