Seawater salinity is one of the basic elements of marine hydrological observation, and seawater conductivity measurement is an important means of salinity measurement. In order to improve the measurement accuracy of seawater conductivity sensor, a seven-electrode conductivity sensor is developed by using lock-in amplifier technology, which can measure the seawater conductivity by detecting the voltage and current signals in the strong noise environment. The comparison experiment was carried out by using seven-electrode conductivity sensor and seabird CTD sensor SBE 37-SI, the indication error of conductivity measurement is within ±0.0003s/m, which can meet the needs of high-precision measurement of seawater conductivity.