Porous silicon (PS) prepared by electrochemical etching method has been studied by positron annihilation lifetime spectroscopy (PALS) and age-momentum correlation (AMOC) measurement in different atmospheres. The longest lifetime component in PALS results is ascribed to the annihilation of positronium in cavities of PS sample. It is found that 80% of the positrons implanted in PS film have formed positronium atoms. PALS results show that the lifetime of ortho-positronium has smaller value when the sample was in oxygen gas medium compared with those in other medium. AMOC results reveal that S parameter of three lifetime components in oxygen are all bigger than that in nitrogen atmosphere. These are probably caused by the oxygen leading to the spin-conversion of positronium atoms.