It was said that the shape of a point target response is just a sinc function after formating in synthetic aperture radar（SAR） systems. But this idea is a mistake actually. The shape of a point target response can be closen as a sinc function shape under some limited conditions. For example

if the time-band width product of a chirp signal is large enough and the spectrum of the chirp signal is close to rectangular

the sequence is long enough to enable more energy of signal being kept down

and the sampling frequency is high enough to ensure less aliasing of the spectrum.Since the last shape is the modulus of a point target

the spectrum of the last shape is extended far away and the spectrum may have more aliasing

and any low-pass filters can not restore the signal satisfactorily. Usually

a liner-interpolation

which is a first-order hold low-pass filter（LPF）

is used to restore signal. It may have more errors than ideal LPF under lower sampling frequency. A higher sampling frequency may make the last shape of point target response more close to the shape of sinc function.Because the spectrum will be extended during the process of demodulation

it may result in aliasing. To change the complex image to modular image

the signal needs upsampling and a low-pass filter before demodulation

and downsampling after demodulation.Specially

more difference may be derived from a two-dimension sinc function shape for the shape of a point target response of a Squint Mode compared with a side-look SAR since its spectrum has more difference from rectangular than a side-look SAR.The reasons of the shape of a point target response differing from a sinc frunction are analyzed

and simulation results have been presented to support the analysis in this paper.