The effects of organic absorbent on the refractive index and photoluminescence of porous silicon microcavities were theoretically investigated by using the Bruggeman effective medium approximation. Experimentally， porous silicon microcavities were fabricated by computer controlled electrochemical etching， and then interacted with the molecules of oil vapor from a rotary pump. We found that the narrow photoluminescence spectra of porous silicon microcavities were sensitive to the adsorption and de-adsorption of the oil vapor molecules， which leads to a 71nm red-shift and the intensity variation in photoluminescence spectrum. The experimental results are qualitatively consistent with the theoretical estimate.