Control of the polarization of light is highly desirable for detection of material's chirality since biomolecules have vibrational modes in the optical region. Here, we report an ultrafast tuning of pronounced circular conversion dichroism (CCD) in the mid-infrared (M-IR) region, using an achiral phase change metamaterial (PCMM). Our structure consists of an array of Au squares separated from a continuous Au film by a phase change material (Ge2Sb2Te5) dielectric layer, where the Au square patches occupy the sites of a rectangular lattice. The extrinsically giant 2D chirality appears provided that the rectangular array of the Au squares is illuminated at an oblique incidence, and accomplishes a wide tunable wavelength range between 2664 and 3912 nm in the M-IR regime by switching between the amorphous and crystalline states of the Ge2Sb2Te5. A photothermal model is investigated to study the temporal variation of the temperature of the Ge2Sb2Te5 layer, and shows the advantage of fast transiting the phase of Ge2Sb2Te5 of 3.2 ns under an ultralow incident light intensity of 1.9 μW/μm(2). Our design is straightforward to fabricate and will be a promising candidate for controlling electromagnetic (EM) wave in the optical region.