Porous silicon films are obtained on highly level doped p-type（100）silicon wafers using electrochemical etching technology.MicroRaman spectroscopy is used to characterize porous silicon in the three different stages: wetting-drying and rewetting

according to the Raman shifts as a result of different factors

the shift of phonon confinement effect resulted from nanocrystalline are separated from the shift that due to strain in porous silicon

a proper expression is found to calculate the stress according to Raman shift.And the stress is calculated on porous silicon surface with different porosity using this express.The results indicate that a larger tensile residual exists on the surface and increases linearly with the porosity

which due to the lattice mismatch between porous silicon and Si substrate and the degree of lattice mismatch higher with the porosity increasing and evaporation.