The preparation of composite powders has been highly valued by researchers in many fields. In order to study the mechanical mixing characteristics of heterogeneous powders with significant density differences, the discrete element method was used to study the mechanical mixing of silica sand, graphite and phenolic resin powder particles with different densities. Two kinds of mixer models with different mixing mechanisms, powder particle model and Hertz-Mindlin with JKR contact model were established. The contact parameters between particles were measured and calibrated by combining experiment and simulation. The force of powder particles in the mixing process, the motion trajectory, the rate distribution, the number of particle contacts and the mixing uniformity were analyzed, and the reliability of the simulation model was verified by experiments. Research has found that the movement of powder particles in the mixer is consistent with the reality under the force of its own gravity, collision and friction of stirring paddle, other particles and mixing barrel. The single spiral conical mixer is more suitable for mixing silica sand, graphite and phenolic resin powder with large density difference, and the relative standard deviation reaches 0.122 after 18 s. The reliability of the model is verified by the mixing experiments of two kinds of mixers and the relative standard deviation of powder calculated statistically. The absolute deviation of the mixing time is less than or equal to 2 s when the relative standard deviation is less than 0.13.