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In synthetic aperture radar (SAR) images, the edge of the shadow is blurred because the radar is moving while the data are collected. In this letter, this problem is expanded on by using the imaging formation perspective. First, an approximate method to represent the imaging quality of the boundary of the shadow region based on the quadratic phase errors (QPEs) is provided for the first time, which built up the relationship between the parameters of the shadow caster and the behavior of the shadow in the SAR image. We notice that the QPE is approximately a linear function of the height of the caster. Second, we deduced the height-dependent phases due to the synthetic aperture process to the raw data, and a novel algorithm called height-variant phase compensation (HVPC) on the complex SAR image data is proposed by compensating the unexpected phases in the azimuth to sharpen the shadow. Compared with the traditional approach called fixed-focus shadow enhancement (FFSE), HVPC removes twice as much of the QPE as FFSE approximately. Experiments on simulation and real data demonstrate the precision and the better effect on shadow enhancement of our work. It is expected that the work in this letter could be some help for the SAR image understanding and application.