This study aims to address the issues of high self-weight, increased brittleness at high temperatures, and limited thermal resistance of ultra-high-performance concrete (UHPC) by introducing lightweight aggregate (LWA) to prepare lightweight ultra-high-performance concrete (L-UHPC). Using a systematic comparative approach, the residual flexural strength, toughness, crack evolution, and microstructural characteristics of UHPC and L-UHPC were investigated at 20°C, 200°C, 400°C, and 600°C. Additionally, acoustic emission technology was employed to elucidate the bending damage mechanism. The results showed that the density of L-UHPC was 1,990.4 kg/m³, significantly lower than that of UHPC (2,442.0 kg/m³). At room temperature, the flexural strength of L-UHPC reached 21.8 MPa, slightly lower than that of UHPC (23.4 MPa). At 200°C, the strength of both materials increased; however, at 600°C, the residual flexural strength of L-UHPC decreased by 17.1 %, a smaller reduction compared to UHPC. Rise time/amplitude–average frequency analysis shows that at temperatures of 400°C and above, the shear crack ratio of UHPC increases to 70.39 %, while that of L-UHPC remains relatively low (66.6 %), indicating superior ductility retention capability and reflecting the mitigating effect of lightweight aggregates on interface damage. b-value analysis further indicates that at 600°C, when the b-values of UHPC and L-UHPC are greater than 0.5 and 0.7, respectively, both materials can maintain a certain level of load-bearing capacity and overall stability. Scanning electron microscopy results validated this trend, showing that at temperatures above 200°C, both UHPC and L-UHPC exhibited cracks in the interface transition zone and fiber–matrix interface, but the damage was more severe in UHPC. In summary, the introduction of LWA effectively enhances the high-temperature resistance of UHPC, providing a theoretical basis and engineering references for the development of high-temperature-resistant lightweight concrete and postfire structural assessment.