In the indirect measurement of Leaf Area Index (LAI), previous studies commonly assumed that the actual LAI can be obtained by subtracting the Woody Area Index (WAI) from the Plant Area Index (PAI), which includes the entire plant. This can be calculated using the equation LAI = PAI - WAI. However, this study, conducted using 3D model simulations and the DHP (Digital Hemispherical Photography) method to measure LAI, discovered that this conclusion may not be correct. The LAI value obtained by subtracting WAI directly from PAI showed significant discrepancies from the true LAI value in the simulated environment. Through the re-derivation and verification of the LAI calculation equation, this study proposes a more reasonable explanation for this discrepancy and presents the following conclusions based on the 3D simulation experiment and hemispherical photography method: using PAI-WAI to calculate LAI leads to overestimation of the values, while the calculation method described in this study obtained more accurate LAI values in the simulation experiments (with higher R ² , lower RMSE and MAE).