Ice water content (IWC) has an important influence on climate change,weather change,weather modification etc. When a millimeter wave radar detects cirrus, particles rotate due to air motion, making back-scattering cross sections inaccurately computed and as a result influencing the IWC which is determined by them. In this paper,models of six non-spherical ice particles were described first and their radiation patterns at specific sizes were then simulated by HFSS software in order to disscuss the difference between different orientation and equivalent spheres. For the case of three different orientations (fixed orientation, horizontal orientation, random orientation), the double-exponential function relationships between particles' maximum size and backscattering cross sections were fitted based on the scattering results of non-spherical ice paritlcles computed by Method of Moment and those of equivalent spherical particles computed by Lorenz-mie theory. Assuming that the mixture of non-spherical ice particles meets the B-H mixing model and the size distribution of cirrus obeys the exponential distribution,which is measured by NASA in 2007,the fixed IWC of cirrus is computed which meets the above two conditions, and radar reflectivities of nonspherical ice paritcles and equivalent spheres at three different orientations,based on the above assumption,also can be computed,which were then put into the known formula IWC-Z to get different results of IWC. The anlysis shows that orientation must be considered when retrieving IWC using radar reflectivity Z,because the IWC retrieved by the equivalent sphere theory would underestimate the actual IWC. These results are significant in accurate retrieving micro-physical parameters of cirrus.