Abstract: There are many kinds of methods to diagnose water demand of crops. This paper studied the crops' physiological electrical indexes. Take maize for example, a sleeve needle type point resistance sensor was used to measure the maize stalk's physiological resistance, and a variable dielectric constant type paraller plate capacitance sensor was employed to measure the maize leave's physiological capacitance. Both of them can immediately and precisely reflect the crop's water condition. The coupled crop water demand information instrument can be applied to diagnose the water shortage and to give hand to automatic water saving irrigation in the course of draught fighting.