The quenching effect of LiF spectrum of C1∏u(v =0, J =53)→X1∑g+(v" = 5, 6: J" =52, 54) for dimer 39K2 by collision of buffer gas Ar was measured experimentally. The relation between the intensities I of LIF spectrum and the pressure P = PAr+PK in beat-pipe oven was deduced by stable collision model. IThe -P plot fitted by this model was in satisfactory agreement with the experimental data. Further analysis of the parameter C1 fitted showed that: the sensitivity of backward detection for heat-pipe oven was 2 times more than that of non-gain fluorescence detection.