Thermal energy storage (TES) is a developing technology with promising applications in renewable energy and waste heat recovery. In this study, we investigate the thermal stability of recently developed nitrate-based form-stable TES materials. Laser-induced breakdown spectroscopy (LIBS) is applied to monitor the release of the Na and K elements in nitrates through chemical decomposition exposing to a simulated high heat flow field, while morphology and thermal performance are characterized by typical ex situ methods. The results reveal that the compatibility between ceramic skeleton materials (SiO2 and MgO) and phase-change materials (NaNO3 and KNO3) plays an important role in the thermal performance of composite phase-change materials (CPCMs). A one-step thermal decomposition reaction of SS (sodium nitrate and silicon dioxide composite) and PS (potassium nitrate and silicon dioxide composite) and a two-step decomposition of SM (sodium nitrate and magnesium oxide composite) and PM (potassium nitrate and magnesium oxide composite) are demonstrated by synchronous thermal analysis (STA). The onset temperature of decomposition (OTD) obtained from in situ LIBS measurements is significantly lower than that obtained from STA.