For the practical needs in the data processing of terahertz radar，the discrete dipole approximation method is used to calculate the backscattering cross-section of non-spherical ice crystals with different shapes. Based on the latest refined ice cloud model， the relationship between the radar reflectivity factor Z_m and ice water content I in 220 GHz is established. The calculation results show that both the shapes of the non-spherical ice crystals and the ice cloud model have a particular influence on the Z_m-I relationship. This study has application value for cloud parameter inversion of mid-latitude cirrus clouds， and it can be helpful for the development of terahertz radar in the detection of clouds.