Digital in-line holography has been found to be a powerful tool in 3D diagnostics of gas-solids flows.A study on particle sizing in gas-solids flows using a digital holographic system was presented.The recorded particle hologram was numerically reconstructed by using wavelet transform algorithm.A 2D interpolation method was used to measure particle image size based on the identification and 3D location of particles，and the real sizes of particles were obtained based on the point spread function theory of numerical reconstruction of particle in-line hologram.Gas-solids flows produced by an annular nozzle and in the upper dilute zone of a cold CFB riser were tested by using a digital in-line holographic measurement system.Glass beads with sizes of 20-100μm were used，and a volume of 10 mm×10 mm×100mm in the flow space was measured.It was found that the particles in the measurement volume were successfully reconstructed.The measurement results of particle size distribution agreed with those obtained by a Malvern sizer，which showed excellent capability of digital in-line holography in particle sizing.