Understanding the mechanisms behind large landslides is crucial for mountain safety, yet remains challenging. Among triggering factors, thermal decomposition has been identified as a key driver in large chemical rockslides (landslides involving chemically active rocks, such as carbonates), with evidence from laboratory and field studies. This study develops a coupled thermal-pore-chemical-mechanical model for unsaturated conditions, applied to the Jiweishan landslide. The model successfully reproduces the observed rise-fall-rise sliding velocity evolution of the landslide mass, with the second surge matching the onset of CO2-generating thermal decomposition. Comparative analyses reveal that higher initial saturation accelerates velocity increase of landslide mass. Furthermore, under unsaturated conditions, friction weakening advances thermal decomposition timing, whereas saturation delays it. This timing difference phenomenon is primarily attributed to the competition between two heat-related variables. Regardless of decomposition timing, friction weakening consistently enhances acceleration.