Bioelectrical impedance is a body's important parameter, which, to a certain degree, reflects the physiological state of the human body. According to the shortage that traditional five-segments model of human body impedance can't satisfy the needs of figuring out human trunk subdivision impedance, the exiting solution of eight-segments bioelectrical impedance module is premised on the basis of symmetrical limbs (symmetrical limbs is not existed in fact). Therefore, an asymmetrical model for measuring the impedance of eight-segments of human body is proposed, two solving equations are used to supplement the impedance relation of the left and right upper limbs and the impedance of the left and right lower limbs by using the five-segments impedance model, solving the problem in the asymmetric eight-segments impedance model,where the resistance is hard to work out. Based on this theory, a portable eight-segments body impedance instrument is designed, an integral design of this instrument is introduced and a contrast experiment with the InBody770 instrument is designed. The results show that the accuracy of the measuring instrument is close to the InBody770 instrument, which meets the needs of measuring the subdivision impedance in human trunk part.