This paper studies the optical characteristics of organic matter,humus composition and form,and humic acid at different planting ages of alfalfa in soils on the Longdong Loess Plateau.Results show that as the year of planting alfalfa goes by,soil organic matter content is gradually increased and its quality is improved.However,the situations of both reclaimed land and abandoned land are opposite to that of alfalfa field.From soil surface to underlying layers,ratio of looseness to tightness in alfalfa field planted for 10～20 years is increased,which means that planting alfalfa mainly affects soil fertility in the soil layer of 20—40 cm.The analyses of color tone coefficient(logK) and relative chromaticity(RF) of humic acid indicate that when the time of continuous cropping is between 5 and 15 years,the quality of humic acid tends to be improved.If the time of planting alfalfa is too long,the quality of humus acid may be reduced,the structure of humic acid may become aging,and the quality of humic acid on abandoned land may be reduced as well.