The invention relates to a method and device for detecting conductivity in an instant electric water heater. The measurement device comprises at least two measurement electrodes for being arranged in a water guiding channel separately; a measurement electronic device for providing and analyzing electrical signals influenced by the conductivity of the water; and a transformer, wherein the measurement electronic device is connected to a primary side of the transformer, and the two measurement electrodes are connected to a secondary side of the transformer, so that the measurement electronic device is allowed to be coupled with the measurement electrodes through the transformer. The measurement electronic device provides a primary-side alternating voltage signal on the primary side of the transformer according to rules, so that a secondary-side alternating current passing through the measurement electrodes and the water is set on the secondary side of the transformer according to the conductivity of the water, and the secondary-side alternating current influences the current of the primary side on the primary side of the transformer; and the measurement electronic device measures the current of the primary side for determining the conductivity. Besides, the invention also relates to the instant electric water heater for heating the water.