The insulator has been applied widely into various fields in our daily life, including medical, industrial, and military components. In high voltage application, the insulator is made by ceramic injection molding process commonly. However, during the fabrication of the high voltage insulator, surface defects such as crack or void are often observed after sintering. From some literatures’ observation, the surface defects could be due to some residual stresses and shrinkage. But how to quantitatively define this type of defect and further to solve it properly is still not completed yet. In this study, we have applied numerical simulation based on Moldex3D and experimental study to discover the phenomena of surface defects in a ceramic injected insulator. The goal is to find what the relation between the surface crack and the volume shrinkage during the formation of the green part for the ceramic injected insulator. Results showed that the filling behavior is quite matched for both numerical prediction and experimental study. Moreover, the numerical discovery of the high volume shrinkage areas are almost matched with the locations of the voids and surface cracks in the real injection parts. Furthermore, various operation conditions are considered for solving the surface crack problem.