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In order to analyze the effect of straw mulching on changing characteristics of soil moisture during seasonal freeze-thaw period, Harbin City was selected as the objective study area, and soil liquid volumetric moisture content at different soil depths (i.e. 20, 40, 60, 100, 140, 180cm) was measured in the field under straw mulching thicknesses of 5, 10 and 15cm in winter, and bare soil was taken as control. Meanwhile, the meteorological data was obtained from automatic meteorological station in the study area. The results showed that straw mulching thickness can retard the temporal inflection point of increasing or decreasing of soil volumetric moisture content between 0cm and 60cm soil depth, the retard effectiveness was more obvious with thicker straw mulching, and the retard effectiveness was more obvious in freezing period than that in thawing period. The snowmelt infiltration was prevented by the straw mulching at the beginning of thawing period, thus the liquid volumetric moisture content appeared temporary peak value at soil depth of 20, 40 and 60cm under the condition of bare soil. However, the liquid volumetric moisture content was lower in bare soil treatment than that in other three treatments due to straw mulching at the end of thawing period. The straw mulching could effectively adjust and remain the liquid volumetric moisture content at soil depth of 0~60cm during freeze-thaw period, meanwhile, the adjustment effectiveness was weakened with the increase of soil depth. Both snowmelt and straw mulching can effectively increase soil moisture, but the increasing capability of soil moisture decreased with increasing soil depth. The order of the increasing capability of soil moisture at soil depth of 0~60cm was 15, 10 and 5cm thickness of mulching straw.