Cement fly ash gravel (CFG) pile composite foundation is an effective and economic foundation treatment approach, which is significant to building a foundation, subgrade construction, and so forth. The present paper aims to investigate the temperature behaviours of high latitudes and low altitude island permafrost when the CFG pile was constructed, in which FEM and field temperature monitoring of permafrost was utilized. The proposed findings demonstrate that the temperature disturbance of permafrost is obvious when the CFG pile was constructed, and the temperature variation of CFG pile and permafrost in FEM has a good agreement with the results of field temperature monitoring. Additionally, the influence of different height subgrade on the temperature field of CFG pile composite foundation was analyzed by FEM. The proposed findings demonstrate that the height of subgrade affects the maximum temperature increase of permafrost and the re-frozen time of permafrost after the construction of CFG pile composite foundation.