Abstract: On the background of target detection for the low altitude radar seeker，influence of the clutter plus thermal noise on the detection performance of the radar seeker is analyzed in this paper.With the assumption that matched filtering，square-law detection，noncoherent integration，cell averaging constant false alarm rate detection and double-threshold detection are employed sequentially for radar seeker detection，mathematical expressions of radar seeker detection and false alarm probabilities in K distributed clutter plus Rayleigh distributed thermal noise environment are obtained by theoretical derivation.Based on this，detection performances of the radar seeker in various circumstances are given by simulations， and comprehensive effect of the clutter plus thermal noise on the detection performance of the radar seeker is analyzed.Analysis results show that radar seeker detection probability decreases as clutter shape parameter decreases and clutter-to-noise ratio increases.Decreasing velocity of the detection probability increases as the clutter shape parameter and clutter-to-noise ratio decrease.With fixed detection probability，the differences of the needed signal-to-clutter plus noise ratio for different fluctuating targets are influenced just a little by the fluctuation degree of the mixed background.