To explore the combustion behavior and decomposition kinetics of the raw rubber under different radiation heat fluxes, the heat release rate (HRR) of natural rubber (NR), ethylene propylene diene monomer (EPDM), butadiene rubber (BR), nitrile-butadiene rubber (NBR) and styrene-butadiene rubber (SBR) were measured via the cone calorimeter. The HRR curves of NR and EPDM under both 15kW/m2 and 35kW/m2 radiation heat fluxes are of the single peak type. However, the HRR curves of BR, NBR and SBR change from the single peak type at 35kW/m2 to the double peak type at 15kW/m2. The formation of the double peak type HRR curves can be explained by the appearance variation and pyrolysis characteristics evolution of specimens before ignition. For BR, NBR and SBR under 15kW/m2 radiation heat flux, the specimen at 5s before ignition can be divided into multiple layers, and the top surface is composed of rigid char like material. Further, at least one of the layers has higher char yield and greater activation energy of decomposition reaction than the upper part of specimen under 35kW/m2 radiation heat flux. The formation of multiple layers and the evolution of pyrolysis characteristics may be related to the fact that the raw rubber stays in the cross-linking reaction temperature range for a relatively long time under 15kW/m2 radiation heat flux. However, the cross-linking effect of NR and EPDM is greatly weakened due to the reversion phenomenon, which prevents the HRR curve at 15kW/m2 from converting to the double peak type.