Abstract: By measuring the water potential of fixed points and sites in different interfaces of the SPAC system of maize field in North China Plain，the resistance of water transferring is calculated．The results show that atmospheric water Potential and leaf water potential have obvious daily variation．The maximum water potential appears before sun rising；the daily atmospheric water potential changes violently，and the soil water potential changes relatively little，mainly under the influence of the soil water content．There exists a grade of water potential that gradually decreases up forward within the plant of summer maize．In the course of water transmission in SPAC system，different interfaces show obvious grades of water potential and great resistance．The resistance from leaf to atmosphere is much more than that from soil to leaf．implying that the resistance of leaf-atmosphere plays a key role in the restriction and adjustment of the dissipations rate and quantity of water from plants．