RF leakage cancellation technology of millimeter wave Frequency Modulated Continuous Wave(FMCW) radar in Foreign Object Debris(FOD) detection system is studied. The reasons of the RF leakage problem in FMCW radar are analyzed, and the influence of the leakage signal on FOD detection system is described. The principle of adaptive filter is analyzed, and then an adaptive cancellation scheme based on Least Mean Square(LMS) algorithm is proposed，combined with characteristics of FMCW radar. The feasibility of the implementation of the proposed cancellation scheme is verified by simulation. The effect of Digital Signal Processing(DSP) response time on FOD system performance is analyzed. The experimental results show that this scheme can achieve a cancellation rate of 45 dB，and realize a high precision cancellation.