The microstructural characteristics of composite binders containing fly ash and (or) tuff powder were investigated by XRD (X-ray diffractometer), TG (thermogravimetry), MIP (mercury intrusion method) and SEM (scanning electron microscopy). Results show that feldspar, hydromica and α-SiO₂ were produced in the hardened paste due to the incorporation of tuff powder, and the other hydration products in the hardened paste of composite binders were basically the same as those of pure cement pastes. The decline of Ca(OH)₂ content in the hardened cement paste containing tuff powder was significantly lower than that of cement-fly ash binary cementitious system. Moreover, the porosity of hardened pastes of complex binders decreased gradually and the pore size was gradually refined with the extension of curing age. The most probable aperture distribution of each sample hydrated for 180 days was mainly concentrated in 4.5 nm-50 nm, and the microstructure of hardened pastes of complex binders was developing in a direction favorable for durability. The morphology effect and micro-aggregate filling effect were caused by the special morphology of tuff granules, which were obvious in the early stage of hydration. The hydration product of the ternary cementitious system containing cement, tuff powder and fly ash were much more than that of the cement-fly ash binary system and the cement-tuff powder binary system with the same dosage. The spherical particles of fly ash were encapsulated by hydration products of tuff powder and cement, making the microstructure of pastes combined closely.