Abstract: Shearing is the basic factor involved in gravity thickening of paste. This work focuses on the influence of pores and throats characteristics on water drainage channel evolution, and determines the proportion of discharged water in tailings thickener bed. Pilot-scale experiment combined with computed tomography (CT) and pore network model (PNM) technology to determine the micropore structure. The maximum ball algorithm is used to analyze the evolution of pores and throats with and without shearing. The results show that the tailings underflow concentration increases from 55.8% to 58.5% under 2 r·min^-1 rake shearing and the porosity decreases from 43.05% to 36.59%, the decrease rate of porosity is 15%. The pore structure can be divided into two types, i.e., "balls" and "sticks, " by the PNM technology. The quantity of "balls" and "sticks" increases by 16.5% and 22%, respectively. However, the average radius of balls decreases slightly in the range of 40-60 μm under shearing. The average radius of sticks decreases from 9.83 μm to 8.58 μm, i.e., by 12.7%. Nevertheless, the length of sticks exhibits only a slight change. The coordination number of balls increases significantly from 25.73% to 44.58% in the range of 5-10 under shearing, and the particles are in close contact. The concept of "the volume ratio of pores to balls" is proposed for the quantitative characterization of the pore structure. The volume fraction of balls decreases from 14.14% to 12.75%, the decrease rate of volume fraction is 9.83%, and volume fraction of sticks decreases from 28.91% to 23.84%, the decrease rate of volume fraction is 17.54%. The volume ratio of balls to sticks increases from 48.91% to 53.48%, and increase rate of it is 9.34%. When the volume decrease of balls is more than that of sticks, the volume ratio of balls to sticks increases. This work reveals the shearing drainage mechanism of unclassified tailings gravity thickening from the perspective of pore structure change, i.e., the drainage is mainly discharged from the throat more than the pore from the tailings thickener bed shear dewatering process.