Abstract：Based on the theory of two-phase homogeneous equilibrium model (HEM), the choking phenomena of gas-powder flow were experimentally studied and theoretically analyzed. Experiments were conducted in a horizontal constant cross-section duct with three kinds of powder: Al2O3, hollow Al2O3 and Al, with different densities, specific heat capacities and thermal conductivities, in order to understand the relationship between the mass flow-rate and the inlet stagnant pressure. Applying the choking principle, the apparent critical pressure ratio and apparent sonic velocity (ASV) of gas-powder flow were determined. Considering the thermal relaxation of the two-phase flow, the effective specific heat capacity (ESHC) of solid phase, the effective isoentropic exponent (EIE) of the mixture and the relative effective specific heat capacity (RESHC) were defined. The experimental results show that, with the increase of the stagnation pressure, the mass flow rate increases as a cubic polynomial function before choking and linearly after choking. The influence of the physical properties of powders on ASV of gas-powder flow is obtained by the comparison between experimental results of different powders. When solid mass loading (SML) is small, ASV is mainly affected by the thermal conductivity of the powder. When SML is large, the density of the powder affects ASV instead. ESHC of the solid phase decreases with the increase of its loading. The thermal conductivity is the main factor to affect ESHC, while density is hardly so. ESHC of the solid phase is very small (<0.5%), and may be ignored. Therefore, the isoentropic exponent of the gas phase can be used as that of the mixture to calculate the ASV of gas-particle flow.