The ignition process of a thermoplastic polymer is investigated numerically regarding to the influence of a periodic radiation source for a given range of frequency. The ignition process is initiated when a solid polymer receives a high radiation heat flux in convective flow environment. The solid polymer undergoes preheating, melting, and pyrolysis with the development of ignition. The combustible gases, as a result of condensed phase pyrolysis process, react with oxygen in the gas phase via combustion reaction. A two-dimensional multi-phase combustion model is utilized to predict the influence of periodic radiation on ignition. The ignition delay and combustion characteristics are examined with respect to the transient patterns of the external radiation sources.