Abstract: In radar signal processing, pulse compression using matched filter can obtain the maximum signal-to-noise ratio, which effectively reduces the influence of noise in radar echo. However, the output of pulse compression has highly sidelobe level. In weather radar detection, due to the large difference in reflection intensity between spatially distributed scattering particles, the echo of weak scattering particles is easily masked in the sidelobe of strongly scattering particles. Therefore, there is a need for ultra-low sidelobe. This paper mainly studies in nonlinear frequency modulation(NLFM) waveform and the minimum integrated sidelobe level(ISL) mismatch filter which can reduce the sidelobe, and analyzes the influence of Doppler shift on its performance. By optimizing the mismatched filter, the performance of the ultra-low sidelobe can be achieved even if the echo has a Doppler shift.