In order to achieve the self-sustaining fusion goal of inertial confinement fusion (ICF), extremely strict requirements for the quality of deuterium-tritium(D-T) ice in the target shell have been put forward. The concentricity between the inner surface of the ice and the target shell is required to be greater than 99.9%, and the root mean square (RMS) roughness of the inner surface of the ice is better than 1 μm. The high-quality ICF target is based on the high-quality D-T ice in the target shell. Single crystal is the best form of D-T ice, and seed crystal in target shell is the basic condition. In this paper, the step temperature control method of gradually reducing the heating rate is used to study the retention of seed crystals in the fill tube, and the nucleation mechanism of retention of seed crystals in the fill tube is revealed. The experimental results show that the use of the fill tube defects can keep stable and single seed crystal, and under the same supercooling, when deuterium tritium seed crystal c-axis and the fill tube are axially parallel, the growth rate is about 1–2 orders of magnitude slower than that when the c-axis is perpendicular to the axial direction of the gas filled tube. The results provide a reference for obtaining high-quality seed crystals, and a basic support for developing the D-T target in China.