Considering application of long time accumulation to the detection and parameter estimation of Linear Frequency Modulated Continuous Wave (LFMCW), a novel method based on the joint window-variant coherent average method and cyclostationary method is proposed to detect LFMCW signal and estimate its parameters. Firstly, the improvement of Signal-to-Noise Ratio (SNR) gain is extracted with window-variant coherent average method based on segmentation coherent. And then, cyclostationary method is utilized to finish the detection and parameters estimation of LFMCW signal. The algorithm realizes long time accumulation with low computational complexity and solves the problem brought by initial time-offset. On the low SNR condition, this method still has better estimation performance. Simulations verify the effectiveness of the method.