A unified gas-kinetic scheme for the monodisperse gas-particle flow (UGKS-D) is constructed. This new scheme is a multiscale method and mainly results from the direct modeling of the flow physics in the scales of discrete cell size and time step. In detail, the gas phase is considered continuous flow, so the gas-kinetic scheme for Navier-Stokes equation (GKS-NS) is used. For the particle phase, the integral solution of the kinetic equation is used in the construction of numerical flux, in which the particle transport and collision are coupled. Thus, there is no requirement of the time step being less than the particle collision time. Furthermore, the momentum transfer between the gas and particle is achieved by the Kli-atchko drag model, while the heat tranfer is described by the Ranz-Marshall correlation. The UGKS-D is first verified by the collision of two solid jets in a channel. Then, we use the UGKS-D to investigate the shock-driven multiphase instability at an initial particle volume fraction of 0.01. It is shown that the particle morphology and circulation calculated by UGKS-D are in good agreement with the multiphase particle-in-cell (MP-PIC) method. In addition, we find out that the particle collisions have dramatic effects on the computed results of the shock-driven multiphase instability. (C) 2019 Elsevier Ltd. All rights reserved.