Based on the 10 year single observation item Ningbo seismic station water temperature and the 6 year same level comparing test data,we compared and analzed the short-term and long-term instrumental stability of the high precision thermometer. It is found that the instrument has long drift phenomenon,and each instrument drift amount is linked to the instrument characteristics,but with the continuation of observation time,drift amount gradually reduces and the phenomenon has little influence on the seismic precursory observation. Unknown steps and pulse phenomena exist in the short term stability,but the frequency and the amplitude are not large,and are difficult to determine if not in parallel observation. Therefore,these problems should be taken into account in the instrument data analysis. Sometimes short-term instruments are very stable,the whole point will remain at the same value for several days or even 20 days.