In this paper are presented the considerations of overall plan of the LFM digital pulse compression radars，and their parameter selection realized in FFT，Which employ the quasi-block-floating point and fixed-point normalized algorithm. These parameters contain the quantized form in FFT computation，word lengths of the A/D converter，the twiddle factor and the butterf1y of the FFT，and so on. Some calculated data and curves are presented by simulated computation via computer，these data can be useful to engineers and designers.