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