A new method for prepering piezoelectric polymer with cellular structure (piezoelectret) is introduced. Their piezoelectric activity and thermal stability are investigated for the fluorocarbon piezoelectret films produced in this method. The quasistatic piezoelectric d33 coefficients up to 2200pC/N are obtained for the fluorocarbon piezoelectret films; d33 coefficients are relatively independent on the applied pressure in the range of 20kPa; comparing to polypropylene piezoelectret film the new fluorocarbon films show not only higher values of d33，but also much better thermal stability; Furthermore，the thermal stability of the fluorocarbon piezoelectret film can be further improved by the process of pre-ageing. The enhanced thermal stability of d33 for such fluorocarbon films is due to the good thermal stability of charges in fluoroethylenepropylene (FEP) and polytetrafluoroethylene (PTFE)，and the dimension stability of such cellular structure. The charge recombination is mainly through the drifting of the detraped charges along the inside surfaces of the cavities when the films are thermally stimulated.