Soil microbiotic crusts play an important role in the tibba ecosystem.By field sampling,some original soil microbiotic crusts were gathered and their physico-chemical properties were analyzed.The results revealed that the longer the sand dunes have been fixed,the thicker and the more biocrusts are formed.The thickness of the crusts under four different types of plant community ranges within 0.3~1.5cm.The fraction of coarse sands in the crusts (0.25~0.05mm) decreased by as much as 30%,and the thicker the microbiotic crusts,the less the coarse sands.But the bulk density of the microbiotic crusts increased with the thickness,and so did the capillary moisture capacity.The reason maybe is the high moisture absorption by biomaterial like lichen,algae,etc in the soil microbiotic crust.However thick microbiotic crusts might hinder water movement.Nutrient analysis of the crusts revealed that as the crusts increased in thickness.SOM,total N,available N increased remarkably,total P and available P increased a little and pH decreased slightly,which illuminated the soil microbiotic crusts had a strong function of enriching soil water and soil nutrients in the Erdos Tibba (semi-arid).