To develop deuterium-tritium (DT) cryogenic targets that meet the inertial confinement fusion (ICF) experiment requirements, the DT crystal seeding growth process needs to be controlled to obtain single crystalline DT-ice, thus reducing the crystal defects formed during crystal growth and improving ice-layering. In this paper, the close-packed hexagonal (hcp) single crystal growth mode has been established through kinetic theory of crystal growth morphology. Experimentally, the target chamber temperature is controlled to within ± 3 mK and the deuterium (D2) crystal growth process can be observed by backlit shadowgraphy. Results show that slow cooling can reduce the crystal defects significantly at the 20–100 Pa conducting helium pressure. When the cooling rate reaches 2 mK/min, two single crystal growth modes are observed with good reproducibility. Experimental results conform with the proposed hcp single crystal growth model. Compared with the results from Lawrence Livermore National Laboratory (LLNL), the methods of D2/DT single crystal growth in the cryogenic target are proposed.