In the separation process on a gas-solid separation fluidized bed, formation of binary mixtures of fine coal and magnetite is inevitable. Using a high-speed camera system (HSCS), the formation of these binary mixtures was observed, and the mixing mechanism was established. The effects of different factors (e.g. gas velocity and size fraction and mixture ratio of fine coal) on the degree of mixing of binary mixture particles were studied using a combination of factor analysis method and orthogonal test method. The results show that the mixing process of fine coal and magnetite plays a crucial role in the adjustment of the bed density. It is clear that the bed density can maintain their identities only when the fine coal and the magnetite are fully mixed. The degree of mixing of binary particles increased with increasing gas velocity while the value decreased with increasing size fraction and mixing ratio of fine coal. The three factors influencing the mixing efficiency showed the following order as: gas velocity, mixture ratio and size fraction of fine coal. Maximum degree of mixing was obtained for the gas velocity of 9.83 cm/s, particle sizes less than 0.15 mm and mixing ratio of 5%. According to the sorting test, the separation efficiency of the gas-solid separation fluidized bed containing binary mixtures is more obvious under the optimal working conditions, i.e., clean coal is obtained with the ash content reduced by 25.67% compared with raw coal and the probable error (E) is 0.13.