To investigate the effect of the hydrogen element on diamond crystallization, hydrogen doped diamond crystals are synthesized at 5.0–6.0 GPa and 1250–1600 °C by adding ferrocene (C₁₀H₁₀Fe) to a system of carbon and Fe-based solvent–catalyst. The essential dependence of diamond morphology and nucleation on the composition of the crystallization medium is established in the P–T diagram. It is found that the growth region of the {111} face becomes wider, while the region of the {100} face becomes narrower and almost disappears with increasing concentration of C₁₀H₁₀Fe. The changing of the diamond growth habits can be explained by the effect of hydrogen from the decomposition of C₁₀H₁₀Fe on diamond crystallization. Fourier transform infrared absorption spectra reveal that the hydrogen-related peaks increase with an enhancement in the ratio of C₁₀H₁₀Fe additive. The observed Raman shift is due to the doping of hydrogen atoms into the diamond lattice. In addition, we find that the change of characteristics of the growth medium of the diamond crystal induced by hydrogen is an important factor for the changes of the synthesis conditions, the growth rate and the nitrogen concentration of the synthesized diamond.