The use of biochar in cement-based materials can improve their basic properties and achieve carbon sequestration while enriching the ways of resource utilization of biomass solid waste. To study the effect of carya cathayensis peels biochar on mortar’s performance, including fluidity, microstructure, basic mechanical properties and thermal insulation, mortar samples whose fine aggregates were partially or completely replaced by carya cathayensis peels biochar were prepared. The experiment results suggest that: 1) the best workability of the mix is found with extra water addition at the rate of 25% of biochar by mass. 2) The highest flexural and compressive strength are found in mortar with 25% biochar-sand-replacement by volume fraction, which has a better interfacial transition zone and paste-wrapping around the aggregates particles, beyond this addition. With the increase of sand substitution rate, the strength of mortar specimens decreases step by step. 3) The thermal conductivity of cement mortar presents a significant decreasing trend with the increase of biochar addition. The native pores of biochar prolong the heat transporting path and exhibit the effect of slowing down thermal conduction which significantly improve the thermal insulation and heat preservation performance of cement mortar.