The mechanism of arching in a moving-bed standpipe with interstitial gas flow has been studied using the theories of particulate media mechanics and non-fluidized gas-solids flow. In order to determine the critical arching radius, which marks the change from arching to flow, a situation where all the powder is in a critical passive state of stress is considered, and the powder in the standpipe is assumed to be composed of arched layers. The differential equations of force balance for a elemental arched layer corresponding to conical, trapezoidal, cylindrical, and rectangular tube shapes were given, respectively. The solutions of these differential equations, which can be used to determine the critical arching span, were also given. The boundary condition for solving these differential equations are that minor principal stresses equal zero at bottom surface of the arched layers. The calculated results of the critical arching span agree well with experimental data and these equations for predicting the possibility of arching can therefore be applied to industrial operation and design to preclude moving-bed flow from arching.