Abstract: To meet the demand for high-precision measurement of ocean temperatures, the long-term stability of domestic Negative Temperature Coefficient (NTC) thermistor elements for ocean temperature measurement was investigated by using the national temperature reference of water triple point and gallium melting point. The calibration method for NTC thermistor thermometers was explored by introducing the thermostatic bath with the isothermal heat pipe technique and adopting the “instantaneous comparison method”. The results showed that the domestic NTC thermistor elements can have a half-year stability better than 1 mK, but long-term evaluation is still necessary. The conventional packaging processes can easily decrease the stability of NTC thermistors, and multiple thermal cycles are an effective way to improve the stability. The number and distribution of calibration points have a great influence on the calibration. But there are high-performance thermometers with little calibration deviation, hardly affected by the distribution of selected points. The analysis of these thermometers can help establish NTC thermistor evaluation standards and shed light on the development of the field of ocean temperature measurement.