The absorption spectra of spirobenzopyran 6-nitroBIPS doped polystyrene thin film with and without UV irradiation were measured. All-optical modulation of 590 nm probe light utilizing the alteration of the absorption of the thin film was studied. The dependence of the optical modulation on the concentration of the dye doped and the intensity of the excitation UV light was also studied. The photochemical quantum yield is 0.24 and the molar absorption coefficient at 590 nm of the photoproduct is 2.0×10 4 Lmol -1 cm -1 according to the theoretical fitting. The time constants of the front line and the after-edge of the modulated light signal were about 131 ns and 18 μs, respectively.