Summary Cement‐based composites is a promising type of structural material, which has prospective applications in relieving the urban heat island effect in summer and melted snow with low energy consumption. However, the major drawbacks of cement‐based composites are heterogeneity, porosity, and brittleness. Porosity and microcrack have considerable influence on the thermoelectric of cement‐based composites applied in large‐scale concrete structures in future. This paper studied in detail the effect of porosity and crack on thermoelectric properties of the cement‐based composite. The proper pores and cracks in the cement matrix are advantageous to enhance the Seebeck effect, but meanwhile it also reduces the electrical conductivity. So combined with Seebeck effect, electrical conductivity and other factors, it can obtain a comparatively low electrical conductivity (0.063S cm−1) of expanded graphite/carbon fiber reinforced cement‐based composites (EG‐CFRC), but EG‐CFRC manifests the maximum thermoelectric figure of merit (ZT) has reached 2.22 × 10−7 when the porosity is 3.90%. With different porosity, the Seebeck effect of prepared EG‐CFRC was strengthened when the crack existed. The effect is most pronounced by a factor of 2 when the porosity is 28.90%. Therefore, based on stabilizing the conductivity, the crack is fittingly made to have a good effect on the Seebeck coefficient. In this work, we studied in detail the prominent effect of porosity and crack on thermoelectric properties of the cement‐based composite. Plenty of pores and cracks exist in cement‐based composites due to its heterogeneous, multi‐phase and brittleness. Based on stabilizing the conductivity, the crack is fittingly made to obviously increase the Seebeck coefficient, a promising method for ameliorating the thermoelectric performance of the EG‐CFRC was obtained. It can obtain the maximum thermoelectric figure of merit (ZT), which has reached 2.22 × 10−7 when the porosity is 3.90%.