To explore motion characteristics of indoor respirable particles, Brownian force on particles is considered in motion probability model. Motion characteristics of 0.01 μm, 0.1 μm and 1 μm particles under conditions of up supply with up return and up supply with side return air forms were simulated with lattice Boltzmann method taking Re as 400, 1 000 and 2 000, respectively. It shows that range of particle spacial distribution increases with Re, and smaller particles affected more obvious by air turbulence and diffusion effect. Mean square displacement (MSD) of particles is inversely proportional to Re and diameter of particles. At same Re, MSD of particles is greater under up supply with side return air form, thus lower suspended particles and higher indoor air quality appears in up supply with side return air form.