A diameter distribution model was constructed to fit the diameter class structure of a mixed forest of spruce and fir trees.

Taking a 1 hm² coniferous and broad-leaved mixed plots in the Jingouling forest farm of Jilin region as the research object, a finite mixed Weibull model was used for fitting the diameter distribution of each plots, and the fitting results were compared with the Gamma model and Weibull model. The results indicated that for most of the mixed forests of spruce and fir in the region, they belong to a unimodal distribution, and from the perspective of shape parameters, they belong to a positively skewed mountainous distribution.

The RMSE (root mean square error) and MAE (mean absolute error) of the finite mixed Weibull model are significantly smaller than those of the Weibull model and Gamma model, indicating a clear advantage; The RMSE error of the finite mixed Weibull distribution decreased by 29.38% and 31.73% respectively compared to the Weibull distribution and Gamma distribution, and the MAE decreased by 28.27% and 36.23% respectively. Regardless of unimodal or multimodal distribution, the finite mixed Weibull model is superior to the Weibull model. From the composition of tree species in various plots, the advantage of the limited mixed Weibull model is greater in plots with a higher proportion of coniferous trees.

It indicates that more finite mixed Weibull models can be used to replace the Weibull model when fitting the diameter distribution of spruce and fir mixed forests.