Abstract: The effective thermal conductivity (ETC) is a key physical parameter for predicting the temperature distribution of concrete structure under fire. Responding to this demand, a multi-scale homogenization method based on the improved Maxwell-Eucken model, considering interfacial thermal resistance and random particle shape, was proposed to estimate the thermal conductivity of cementitious composites after high temperature. Firstly, the thermal conductivity and porosity of mortar, high performance concrete and polypropylene fiber reinforced concrete with thermal treatment at different temperatures (20, 60, 150, 300, 450 and 600℃) were measured and a part of experimental data was used to calibrate the proposed method. Finally, the method was verified by good agreement between experimental data and numerical results of concrete with different heating temperatures and different fiber contents and size. The results show that: The particle shape (fiber length) has a marginal effect on the ETC; The interfacial thermal resistance (ITR) caused by particle and matrix debonding has a significant effect on the ETC, and the ITR coefficient is in direct proportion to the heating temperature; The polypropylene fiber melts and evaporates at high temperature and then the ‘tunnel crack’ filled with dry air forms a thermal resistance. In addition, the melting and evaporation of soft fine polypropylene fiber adhering on the coarse aggregate surface enhances the ITR effect between aggregate and mortar.