Based on the spiral descending model of chaff， the effect of both rotation and translational motion of chaff on radar echo of chaff clouds was studied and the full-polarimetric spectral characteristics of radar echoes from chaff clouds were investigated by using statistical method. Firstly， spiral descending model of chaff was described， and the Sinclair matrix of a dipole with rotation was given. Based on the above analysis， auto correlation and cross-correlation functions of the Sinclair Matrix elements were investigated and further power spectrum and cross-power spectrum were derived. At last， some numerical results of radar echo spectra for randomly oriented and horizontally oriented chaff clouds were presented.