Aimed at the problem of weak radar echo of small unmanned aerial vehicle targets and difficult target detection. Long-term coherent integration method under the LFMCW(linear frequency modulation continuous wave) radar was studied. By deriving the expression of LFMCW radar echo, an Radon-Fourier transform implementation method based on Chirp-Z transform of time-domain beat signal was proposed. The computational complexity of this method was evaluated and compared with the method implemented in frequency domain. Simulation and measured data verified the effectiveness of this algorithm for the coherent integration of weak targets under LFMCW radar.