In order to obtain hydroxypropyl methyl cellulose (HPMC)/cement-based composite porous thermal insulation materials with good performance,the effects of different amounts of fly ash cenospheres(FACs) on the mechanical properties,thermal conductivity,dry density and water absorption of HPMC/cement-based composite porous thermal insulation materials were studied. At the same time,SEM,XRD and image analysis software were used to characterize the pore structure characteristics of composites. The results show that with the increase of FACs content,the thermal conductivity decreases first and then increases,and the compressive strength increases first and then decreases. When the FACs content is 9.0%(mass fraction),the thermal conductivity of composites is the smallest,which is 0.094 W/(m·K). Under this blending ratio,the 28 d compressive strength is 1.58 MPa,and the dry density is 418.9 kg/m3. When the FACs content is 18.0%,the compressive strength of composites is the best,the 28 d compressive strength is 2.15 MPa,the thermal conductivity is 0.101 W/(m·K),and the dry density is 431.0 kg/m3. With the increase of FACs content,the proportion of pores less than 300 μm in composites increases gradually，and the average pore size shows a decreasing trend.