The stored energy provided by the fabric assemblies will greatly influence the thermal protection performance (TPP) of firefighters’ protective clothing under low-intensity radiation heat exposure. In this study, two test methods, namely radiant protective performance (RPP) and stored energy test (SET), were used to investigate the TPP of the fabric assemblies. The results indicated that TSET value was lower than TRPP value because of the release of the stored energy in the fabric assemblies after heat exposure. Increasing the fabric layer numbers, air gap between the fabric assemblies would increase the time of TRPP and TSET, indicating that the thermal stored energy weakened the TPP of the firefighters’ protective clothing. Moreover, the TRPP and TSET of the fabric system would be increased when the moisture barrier was cut in the fabric combination system. These findings suggested that stored energy should be considered in analyzing the TPP of fabric assemblies..