A theoretical model of viscosity in gas-solid separation fluidized beds is established according to the two-phase flow theory of fluidized beds. After comparing theoretical and measured values, the correlation coefficient between the two is as high as 0.99, showing that the model has good predictability for the viscosity of fluidized beds. Meanwhile, the viscosity and its influencing factors were studied using a Brookfield viscometer. The study shows that smaller medium particles (0.074-0.15 mm) can reduce the viscosity of fluidized beds, but they will aggravate the viscosity fluctuation at more than 5 wt% addition, which is unfavorable to the stability of fluidized beds. In addition, in the actual separation process, the external factors (such as moisture and coal powder content) also affect the viscosity of the fluidized beds. Increasing the moisture increases the viscosity of the fluidized bed, whereas coal dust has the opposite effect. In order to ensure the stability of the fluidized bed, the bed moisture content should be controlled below 1 wt%, while the content of coal powder should be limited below 5 wt%. Based on separation tests, reducing the viscosity will improve the separation performance of a fluidized bed at the proper fluidized gas velocity, with the lowest possible error E-p of 0.085. (C) 2018 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.