Biomass is a renewable resource that can significantly reduce the energy dependence on fossil fuels. In this paper, numerical simulation of a three-dimensional bubbling fluidized gasifier is carried out via the multiphase particle-in-cell approach to explore the effect of particle size distribution (PSD) on the gas yield of biomass gasification process. The impacts of PSD width on the gas production, the mole fraction of gas species combined with the gas temperature are investigated. The results show that enlarging the PSD width increases the total gas yield, the mole concentration of combustible gases (CH4, CO and H-2) and also the carbon conversion efficiency, demonstrating the promoting effect of PSD width on improving the gasification performance. Regarding the axial distribution, a large mole fraction of combustible gases can be observed for the system with large PSD width. Large mole fractions of combustible gases and O-2 are mainly distributed in the dilute region and the dense region, respectively. The results obtained provide new insights regarding the effect of particle size distribution of the bed material on the gasification performance of the bubbling fluidized gasifier, which is useful for the design and operation of this apparatus. (C) 2019 Elsevier Ltd. All rights reserved.