Based on observations on field soil water,the growth and yields of winter wheat under sub irrigation condition,this paper studies the soil water dynamics of winter wheat field and water saving mechanisms.The analytical results showed that the field soil water dynamics of winter wheat field under sub irrigation condition had close correlations with irrigation,the growth of winter wheat and weather conditions.The results also showed that the consumption of irrigation water under sub irrigation condition was obviously reduced,and the sub irrigation was propitious to winter wheat growth and yield formation.During the whole growth period of winter wheat,the water consumed by sub irrigation was reduced by 44.4mm,a decrease of 1200 m3/ha of irrigation water than that by spray irrigation.The final yield increased by 714 kg/ha under sub irrigation condition.The WUE was 21.11kg/(ha·mm),which was 4.49 kg/(ha·mm) more than that under spray irrigation condition.The water saving mechanisms of sub irrigation for winter wheat are:(a)The humidity of soil surface (0～20cm) under sub irrigation is much less than that under spray irrigation during the whole growth period.So the non effective physical evaporation of soil water is greatly decreased under sub irrigation condition.(b)Under sub irrigation condition,because the soil surface (0～20cm) is drier,the wheat root systems are forced to extend to deep level to sop up water from medium soil levels (20～120cm),which have abundant soil water supplied by sub irrigation.As a result,the water utilization efficiency from the medium soil levels is increased.