Abstract: Aims To elucidate the rainfall redistribution characteristics among different plantation types in the mountainous regions of Beijing—particularly the interception mechanisms of mixed forests with structurally complex canopies—this study conducted a quantitative analysis of throughfall, stemflow, and canopy interception based on field observations and simulations using the modified Gash model. The primary aim was to assess the model’s applicability under heterogeneous canopy structures and to provide a scientific foundation for regional water management and vegetation configuration optimization. Methods Field measurements were conducted during the 2021 growing season (June to September) in the mountainous region of Beijing, targeting pure stands of Platycladus orientalis, Quercus variabilis, and their mixed forests. Meteorological and gross rainfall were collected alongside in-situ measurements of throughfall, stemflow, and canopy interception. The modified Gash model was applied to simulate canopy interception across forest types, followed by a parameter sensitivity analysis to evaluate model applicability. Important findings (1) Light rainfall events dominated the study period, accounting for 73.5% of gross rainfall. The observed mean proportions of throughfall, stemflow, and canopy interception were 74.15%, 1.89%, and 23.96% for Platycladus orientalis; 71.70%, 2.25%, and 26.23% for Quercus variabilis; and 68.74%, 2.56%, and 28.70% for the mixed forest, respectively. Throughfall and stemflow were significantly positively correlated with gross rainfall, while canopy interception exhibited a positive power-law relationship with rainfall. (2) The model-simulated canopy interception rates were 21.92%, 23.13%, and 26.47% for Platycladus orientalis, Quercus variabilis and the mixed forest, with relative errors of 8.52%, 11.82%, and 7.75%, respectively. The model demonstrated good performance across all forest types, with the highest accuracy observed in the mixed stand. (3) Sensitivity analysis indicated a consistent ranking of parameter influence on model output: average evaporation rate > average rainfall intensity > canopy storage capacity > canopy cover > trunk storage capacity > stemflow coefficient. In summary, structural differences among forest types led to distinct rainfall redistribution patterns under similar climatic conditions. The modified Gash model proved applicable to typical plantation and mixed forests in the mountainous areas of Beijing and can effectively support studies on forest water balance in the region