The rate control of ring closure reaction of photochromic chalcone-epoxy doped with spiropyran dye was achieved successfully by virtue of [2pi + 2pi] photocycloaddition between the chalcone units of host polymer and photopolymerization of epoxy group at the chain ends, which is expected to control the free volume surrounding photochromic moiety. The kinetic measurement was conducted to prove the improved photostability of the merocyanine chromophore that is colored species of spiropyran. The chalcone-epoxy polymer system that contains chalcone group in the repeating unit of the main chain was proved to retard ring closure reaction of merocyanine significantly by the effective steric hindrance after UV irradiation than that of methacrylate polymer containing chalcone in the side chain.