We proposed a dual-period phase grating using a polymer-stabilized blue phase liquid crystal (PS-BPLC). High efficiencies of 38% for the short-period phase grating and 30% for the long-period phase grating have been achieved because of the electric-field-induced rectangular-like phase profile of a PS-BPLC, which agrees quite well with the simulation results. The diffraction angle and the diffraction efficiency can be alternatively tuned by a bias voltage and intensity of applied voltage. The diffracted light of each diffraction order is elliptically polarized, and its ellipticity is larger than that induced by using the conventional nematic LCs. Such a device also shows sub-millisecond response time and holds great potential for photonics applications.