This paper carries out the research on the critical conditions of ferroelectric domain switch with different porosity and pressure. A micromechanics model is established with the influence of external electric field, stress load, domain wall energy and depolarization energy, etc. The results show that the domain switch critical dimension and nucleation energy decrease fastly in the initial stage and then decrease slowly as the reversed electric field evenly increases. The critical dimension and nucleation energy with different porosity and pressure are studied, too.