Porous silicon (PS) samples treated by chemical etching were illuminated with laser uninterruptedly in atmosphere and oxygen, respectively. The photoluminescence (PL) band of porous silicon exhibited a continuous blue shift with the time of illumination, at last it reached a stationary value. In vacuum, the sample was treated in the same way as mentioned above. But its PL band showed no shift. X-ray photoelectron spectroscopy showed that no SiO_2 was detected in the two PS layers which were illuminated in atmos- phere and oxygen, respectively. Combining it with the infrared absorption spectra measurement it is suggested that the blue shift of the PL band was attributed to the replacement of Si atom on the inner walls of PS by oxygen atom and the formation of the Si -O-Si structure.