The invention relates to a hydrogenation method for preparing diesel oil fractions from oil and fat raw materials. The oil and fat raw materials are successively reacted with a hydrotreating catalystI and a hydrotreating catalyst II. Liquid hydrocarbon, water and gas stream are be obtained by separating hydrotreating reaction effluents, the liquid hydrocarbon is reacted with a hydroisomerizationcatalyst by contacting after gas extraction, and hydroisomerization reaction effluents are separated and fractionated to obtain the diesel oil fractions. The prepared diesel oil fractions are sulfur-free, low in freezing point and high in cetane number, so that the diesel oil fractions are high-quality diesel oil products, and can be used by mixing with fossil-based diesel oil or used as additivesto improve the cetane number of diesel oil.