Gas/liquid-solid two-phase flow is ubiquitous in chemical engineering.This paper theoretically and experimentally analyzes the mass conservation of solid phase in a CFB riser.It is revealed that the time-average particle velocity and arithmetical-average particle velocity are misused in the traditional viewpoint of two-phase flow.The introduction of solid volume fraction in a two-phase flow actually denotes an assumption that the discrete particles are treated as a continuum.Then the misuse of time-average particle velocity and arithmetical-averaging appears inevitable.The physical meaning of time-average particle velocity in a gas/liquid-solid two-phase flow is discussed on a microcosmic scale.An equation for computing the time-average particle velocity in a discrete two-phase flow is given.Furthermore，a proper approach is proposed to analyze the mass conservation of discrete particles.