In order to improve the accuracy of soil moisture measurement by T-TDR (Thermo-time domain reflectometry) probe because the needles of T-TDR were too short, this paper presented a new TDR waveform analysis method named LFF (Low frequency filter). The TDR waveform was transformed by fast Fourier transform algorithm, and then the spectrum of TDR wave was obtained. The LFF transformation could be realized by filtering the low frequency of TDR wave spectrum and transforming it by inverse fast Fourier transform. Four kinds of soil samples prepared by five different volumetric water content with twenty specimen, were used in the experiment. Through comparing the LFF method and the traditional WINTDR method with the oven drying method respectively, LFF method showed better results. The correlation coefficient of LFF with the oven drying method was R2=0.9835. LFF method could improve the soil moisture measurement accuracy of T-TDR and simplify the test validation procedures and improve the test efficiency.