Piezoelectricity of non-polar space charge electret, porous PTFE film with unidi rectional mechanical stretch was studied. Piezoelectricity of double-layer stack s of porous PTFE and non-porous polymer films (such as PTFE, PI, FEP and PCTFE) with space charge layer were investigated. The results of preliminary investigat ion pointed out that under the optimal poling conditions, piezoelectric coeffici ent d33 of porous PTFE with external electrode was up to 186 pC/N whi c h is close to the value of ferroelectric ceramics PZT, and was about one order o f magnitude that of ferroelectric polymer PVDF. Outstanding thermal stability of piezoelectricity for soft PTFE porous film was reported. Influences of poling parameters such as poling time, corona electrical field and charging temperature etc, as well as charge density stored in the material on piezoelectricity were investigated. The influences of charging parameters on piezoelectricity were also explained by means of charge dynamic character and the material property.