Abstract To address the mixing uniformity of multiple organic hydraulic (MOH) mixtures in a continuous mixer, three types of mixing parameters and their coupling effects were studied by the discrete element method (DEM) and response surface methodology. To achieve the research goal, only one parameter was selected for each type of parameter, and the corresponding model was established. Numerical simulations and optimization were implemented. A three-level, three-factor Box-Behnken Design method combined with response surface methodology was applied for the numerical design. The influence of the parameters on the mixing uniformity of the mixture was analyzed by analysis of variance (ANOVA). The ANOVA results show that the rotation speed, the installation angle, the filling ratio and the coupling between the rotation speed and the filling ratio have a significant effect on the mixing uniformity of the mixture, that the rotation speed and the filling ratio have the strongest effect on the response, and that of the fitting model of the mixing uniformity can fit the simulation data well. The coupling effect results show that the influence of coupling between the revolution and installation angle on the mixing uniformity is consistent with that between the filling ratio and installation angle and that the coupling effect between the rotation speed and the filling ratio is different. It is also found that the optimal parameter range under one factor is different from that under multivariable coupling. The optimization results show that when the discrete coefficient is the smallest, the optimal combination of the parameters is a revolution of 350 r/min, an installation angle of 25°, and a filling ratio of 70%. The experimental results are consistent with the optimization results, which indicate the correctness of the parameter optimization results.