The characteristics and mechanisms of stress fluctuation are not fully understood, especially for the large fluctuation at the cylinder/hopper transition. In this paper, we performed wall normal stress tests in a laboratory-scale silo which consisted of the cylinder section and hopper section. The amplitude and period characteristics of wall normal stress fluctuation at the top of the hopper were investigated. The result shows that the peak of the stress fluctuation and the maximum of the fluctuation range at the hopper top can reach 1.60 times and 0.64 times of the theoretical value, respectively. The amplitude of the stress fluctuation at the hopper top is independent of the outlet diameter, while the period of the stress fluctuation decreases with the increase of the outlet diameter. Furthermore, the stress-state transition and the shear zones in bulk solids are reasonably responsible for the periodic stress fluctuation at the hopper top. (c) 2020 Elsevier B.V. All rights reserved.