An infrared-frequency polarizing reflectarray is designed with a phase-amplitude synthesis method at 4μm wavelength. The proposed phase-amplitude synthesis method is based on the optimization of the axial ratio of the steered beam through the reflection phase and amplitude of reflectarray elements. The proposed reflectarray reradiates an incoming slant linearly polarized wave into a circularly polarized reflected wave with an axial ratio less than 0.8dB in the main beam direction.