【Objective】 To accurately predict the crown length of Larix olgensis and Fraxinus mandshurica trees mixed by rows with four different mixing methods (1∶1, 2∶2, 3∶3, 5∶5), crown length models for L. olgensis and F. mandshurica were developed using simultaneous equation models. 【Method】 The data for sample trees of L. olgensis and F. mandshurica in mixed stands were collected from 54 permanent sample plots in the Shangzhi Administrative Bureau of National Forestry Centers in Heilongjiang Province. The best crown length model was selected from three nonlinear basic models, and the Mitscherlich model was selected as the basic model of the height-diameter model. The mixed proportion (Z_i) and the tree position (K) in mixed rows were taken as dummy variables. Other tree variables, as well as stand variables and competition factors were also added. Based on the optimal crown length and height-diameter model, the simultaneous equations model of the crown length and height were developed, respectively. The nonlinear seemingly uncorrelated regression (NSUR) method was used to estimate the parameters, and the fitting effects of the models was evaluated. 【Result】The ratio of the total tree height to DBH was negatively correlated with the crown length of L. olgensis, and the ratio of tree height to dominant tree height was positively correlated with the crown length of L. olgensis. The height-diameter ratio was negatively correlated with the crown length of F. mandshurica and the ratio of tree diameter at breast height to dominant tree diameter was positively correlated with the crown length of F. mandshurica. The average stand height of L. olgensis was positively correlated with the height of L. olgensis, while the dominant tree height of F. mandshurica was positively correlated with the height of F. mandshurica. For the simultaneous equations, the adjusted coefficients of determination ( R_{\mathrm{a}}^2 ) of the crown length and height of L. olgensis were 0.478 1 and 0.821 6, respectively. For the simultaneous equations, the R_{\mathrm{a}}^2 of the crown length and height of F. mandshurica were 0.395 8 and 0.752 9, respectively. 【Conclusion】 Constructing a simultaneous equation system for the crown length and tree height not only has good fitting effects and prediction accuracy, but also solves the problem of the inherent correlation between the crown length and height. Therefore, the crown length models established in this study can accurately predict the crown length of L. olgensis and F. mandshurica in the rows of mixed plantations, thereby providing a basis for a further study on the crown structure in mixed plantations.