This paper analysis the developing of expendable conductivity temperature depth measuring system(XCTD)and introduce its principle of measuring about temperature,salinity and depth of ocean.Some key techniques are put forward.According to the real needs of XCTD,conductivity sensor with high sensitivity is designed by principle of electromagnetic induce,the ocean conductivity from induced electromotive force has been calculated.Adding temperature correction circuit would help to reduce error of conductivity measurement because of sharply changing temperature.Advanced temperature measuring circuit of high precision and the constant current source is used to weaken effect of self-heating of resistance and fluctuation of the source.On respect of remote data transmission,LVDS is a good choice for the purpose of guarantee the quality of data transmitted and the transmission distance is reaching to thousand meters in the seawater.Modular programming method is also brought into this research aimed at improve the stability,reliability and maintainability of the whole measuring system.In February,2015,the trials in South China Sea demonstrate that the developed XCTD realize effective measurement at a speed of 6 knots and detection depth at 800 m.The consistency coefficient of the acquired data is greater than 0.99 and the success rate of probe launching is above 90%. This paper analysis the developing of expendable conductivity temperature depth measuring system (XCTD) and introduce its principle of measuring about temperature, salinity and depth of ocean. Home key techniques are forward. According to the real needs of XCTD, conductivity sensor with high sensitivity is designed by principle of electromagnetic induce, the ocean conductivity from induced electromotive force has been calculated. Adding temperature correction circuit would help to reduce error of conductivity measurement because of sharply changing temperature. Advanced temperature measuring circuit of high precision and the constant current source is used to weaken effect of self-heating of resistance and fluctuation of the source. On respect of remote data transmission, LVDS is a good choice for the purpose of guarantee of quality of data transmitted and the transmission distance is reaching to thousand meters in the seawater .Modular programming method is also brought into this research ai med at improve the stability, reliability and maintainability of the whole measuring system. February, 2015, the trials in South China Sea demonstrates that the developed XCTD realize effective measurement at a speed of 6 knots and detection depth at 800 m. The consistency coefficient of the acquired data is greater than 0.99 and the success rate of probe launching is above 90%.