To image a large-size object with a high spatial resolution in a kiloelectronvolt (keV) X-ray range, a method is proposed to analyze and simulate the imaging of an extended X-ray source by a Fresnel phase zone plate (FPZP), based on the translational invariance of the point spread function in a 1 mm square area on the objective plane. Using this method, the imaging of an extended source of a different size is simulated under a typical experimental condition of image-to-source magnification of 10 for an FPZP of an outmost zone width of 0.35 μm. The results show that the image contrast decreases with the increase of the source size, and the zeroth-order and the minus first-order diffractions of the FPZP contribute mainly to the image background enhancement and the contrast decrease. The spatial resolution to the objective plane is also found to be reduced. For a 1-mm-square-shape source with a sinusoidal-distribution intensity modulation of contrast 1, the image modulation contrast is below 0.4, and the spatial resolution is 0.75 μm.