Due to the satisfactory property and high productivity, heat-cured concretes have been widely used in engineering practice. However, heat curing process also brings some drawbacks that are detrimental to the long-term property of this material. To address this issue, lightweight fine aggregate (LWFA) was employed to provide internal curing (IC) for a heat-cured mortar (HCM) blended with fly ash (FA). The influences of LWFA on the interior relative humidity of HCM and the reaction environment and behavior of FA were measured. It was found that IC of LWFA could mitigate the drop of interior humidity and enhance the reaction degrees of cement and FA. This contributed significantly to the microstructure densification of HCM, higher compressive strength and better resistance to chloride ion. The results indicate that LWFA benefits to enhancing the efficiency of FA in a heat curing system and the combination of LWFA and FA contribute to improving the long-term property of HCM.