For multiple-phase flow measurement, digital in-line holography has the advantages of three-dimensional （3D）and non-contact. It has been found to be a powerful tool in 3D diagnostics of particle field. A study on large and small particles in gas-solids flow field using a digital holographic system is presented. Firstly, the numerical simulation is conducted to study the precision influence of small particles measurement when large particle is existed in the flow field, and the capability of holography measurement on small particles. Secondly, the recorded particle hologram is numerically reconstructed by using wavelet transform algorithm. The size and spatial distribution of particles are obtained based on identification and 3D location of particles. In the experiments, by using digital in-line holographic measurement system, the particle fields are tested. It is found that the particles in the measurement volume are successfully reconstructed, and the measurement results of particle size distribution agree with the actual parameter. This indicates the feasibility that digital in-line holography is used to measure particle sizing and 3D position with different particle sizes and concentration distribution in multiple-phase flow.