Abstract: Ultra low temperature refrigerators are widely used in biological research, clinical medicine, etc. The accuracy of the temperature in a refrigerator directly affects the quality of the samples stored in the refrigerator. Regular calibration of refrigerators is needed to ensure the reliability of their performance. Considering actual situation of on-site calibration, this paper discusses the common problems in calibration of ultra-low temperature refrigerators, such as the selection of calibration instruments, calibration methods, safety protection measures, and gives relevant suggestions.