A hydrogen liquefier is a device used to convert gaseous hydrogen into liquid hydrogen. It employs helium reverse Brayton cycle refrigeration principles and ortho-para hydrogen continuous conversion technology. It is suitable for helium refrigeration cycle hydrogen liquefaction systems below 2.5 tons, with a liquefaction capacity of 200-2500 kilograms per day. The hydrogen purity can reach 99.9999%, with an ortho-hydrogen content of ≥95%. Large-scale hydrogen liquefaction systems use hydrogen gas refrigeration cycle principles and high-speed heavy-duty hydrogen gas turbine expander technology, suitable for 5-30 ton hydrogen liquefaction systems with a liquefaction capacity of 5-30 tons per day.