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 Zm and ice water content I in 220 GHz is established. The calculation results show that both the shape of the non-spherical ice crystals and the ice cloud model have a particular influence on the Zm-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.