Abstract: The definition and partial character of fractional ambiguity is introduced. The LFM-delay characteristic of fractional ambiguity is investigated for the echo which phase has thrice-order term. A new method based on the difference of LFM-delay fractional ambiguity model for moving weak target detection in sea clutter is proposed. In the method the FRAF model of the signal and LFM-delay signal are differenced. Partial clutter can be differenced effectively and the FRAF model of signal can not be differenced at the same time. The signal to clutter ratio and the detection performance can be increased by the method. At last the method is Experiment using the IPIX radar data shows that the method has remarkably advantage in increasing the difference of the peak value of the signal and the peak value of the clutter in the FRAF domain, and increasing the signal to clutter ratio compared with fractional ambiguity. Biparametric constant false alarm rate is introduced in the paper. Setting right CFAR detection threshold, the target can be better detected by the method of the paper.