The influence of birefringent light scattering on in-line optical transmittance model depended on uniaxial hexagonal crystal structure transparent ceramics using ZnO as research object has been established based on Mie theory and its developed approximation Rayleigh-Gans-Debye scatting theory. Theoretical calculation indicates that in-line optical transmittance of ZnO transparent ceramics improves obviously with the decrease of grain size and increase of orientation. ZnO transparent ceramics which meet the microstructure requirements of the model were effectively controlled by slip casting process under a strong magnetic field and designing SPS sintering parameters. Corresponding results show that the in-line optical transmittance of non-textured ZnO transparent ceramic increased from 5.1% to 12.9% at 600 nm as grain size decreased from 1.72 μm to 0.35 μm while that of textured ZnO transparent ceramic (mean grain size 0.66 μm) was improved greatly from 21.6% to 36.6% at 600 nm due to increasing orientation factor to 24.7% calculated from XRD data. Based on these data, it is found that the calculation results of theoretical model match well with the experiment results.