Abstract: Freshness is a key quality attribute of fish and fish products, which can be quickly evaluated from its electricity characteristics. Based on bio-impedance theory, a bio-impedance measurement system of fresh water fish was constructed. The system adopted self-made sensor based on four-electrode structure and sweeping frequency method to research the bio-impedance characteristic along the lateral line and vertical to the lateral line of crucian at 25℃, meanwhile the chemical index TVBN value was measured through semimicro-fixation of nitrogen method. The results indicated that the composite electrode at the excitation frequency of 591 Hz was suitable for the experimental research due to its stable changes of bio-impedance characteristic associated with the decay time. The mathematics model under 591 Hz between bio-impedance characteristic and TVBN was ascertained. When measured from vertical to the lateral line, the gill impedance is less than 91.91Ω, and the gill phase angle is less than 5.06°, otherwise, when measured along the lateral line, the gill impedance is less than 99.61Ω, and the gill phase angle is less than 4.94°, which indicates the spoilage of fish and it is not edible.