Fourier single-pixel imaging (FSI) is an efficient single-pixel imaging method of obtaining high-quality (resolution/signal-to-noise ratio) 2D images, which projects sinusoid patterns on the object and reconstructs the image through reflected light. The typical system of FSI consists of a single-pixel detector and a digital projector. However, the defocusing of the projector lens blurs the projected patterns, which results in reduced imaging quality. In this work, we propose the projector-defocusing rectification for FSI, which optimizes projector defocusing for the first time. The proposed method rectifies Fourier coefficients using the amplitude ratio between original and defocused patterns, which we can acquire through a controlled experiment on a whiteboard. The enhancement of imaging quality in imperfect circumstances is demonstrated by simulations and experiments.