A method for detecting electrical impedance of an imaging target, the method comprising steps of: a) sending a square wave excitation current signal to the imaging target through a pair of exciting electrodes for generating a response voltage signal; b) buffering and amplifying the sampled response voltage signal by a first operational amplifier and a second operational amplifier each adapted for neutralizing an impedance influence caused by contact between the electrodes and skins of the imaging target; c) driving the buffered and amplified response voltage signal through a first high-pass filter RC circuit and a second high-pass filter RC circuit by the first operational amplifier and the second operational amplifier for filtering out interference caused by power frequency and polarized voltage of the electrodes; d) driving the filtered response voltage signal through a differential amplifier circuit for transforming the filtered response voltage signal to a single-ended signal and amplifying the single-ended signal to obtain an amplified single-ended signal; and e) transforming the amplified single-ended signal to a digital signal by an A/D circuit having high-speed and high accuracy for calculating and obtaining information of the electrical impedance by demodulating; characterized in that: the response voltage signal is sampled once when the response voltage signal is at high level and once when the response voltage signal is at low level for every cycle and a first sample V 1 and a second sample V 2 are obtained respectively; a period of respective sampling at high level is supposed as t p 1 = t 1 T / 2, wherein t 1 is a period between a time of respective sampling at high level and a time of a nearest rising edge of the square wave excitation current signal; correspondingly, a period of respective sampling at low level is t p 2 = t 2 T / 2; wherein t p 1 = t p 2; information of the electrical impedance is obtained by calculating and demodulating difference V z = V 1 - V 2 between the first sample V 1 and the second sample V 2; and an average value of the V z from a plurality of the cycles is taken as a final result. a) sending a square wave excitation current signal to the imaging target through a pair of exciting electrodes for generating a response voltage signal; b) buffering and amplifying the sampled response voltage signal by a first operational amplifier and a second operational amplifier each adapted for neutralizing an impedance influence caused by contact between the electrodes and skins of the imaging target; c) driving the buffered and amplified response voltage signal through a first high-pass filter RC circuit and a second high-pass filter RC circuit by the first operational amplifier and the second operational amplifier for filtering out interference caused by power frequency and polarized voltage of the electrodes; d) driving the filtered response voltage signal through a differential amplifier circuit for transforming the filtered response voltage signal to a single-ended signal and amplifying the single-ended signal to obtain an amplified single-ended signal; and e) transforming the amplified single-ended signal to a digital signal by an A/D circuit having high-speed and high accuracy for calculating and obtaining information of the electrical impedance by demodulating; the response voltage signal is sampled once when the response voltage signal is at high level and once when the response voltage signal is at low level for every cycle and a first sample V 1 and a second sample V 2 are obtained respectively; a period of respective sampling at high level is supposed as t p 1 = t 1 T / 2, wherein t 1 is a period between a time of respective sampling at high level and a time of a nearest rising edge of the square wave excitation current signal; correspondingly, a period of respective sampling at low level is t p 2 = t 2 T / 2; wherein t p 1 = t p 2; information of the electrical impedance is obtained by calculating and demodulating difference V z = V 1 - V 2 between the first sample V 1 and the second sample V 2; and an average value of the V z from a plurality of the cycles is taken as a final result.