Two-stage cascade refrigeration system, control method thereof and freeze thawing machine

The invention relates to the technical field of biological refrigeration, and discloses a two-stage cascade refrigeration system which comprises a high-temperature-stage refrigeration circulation loop, a high-temperature-stage refrigeration circulation loop and a low-temperature-stage refrigeration loop. The low-temperature-stage refrigeration circulation loop is connected with the high-temperature-stage refrigeration circulation loop through the cascade plate heat exchanger and exchanges heat with the high-temperature-stage refrigeration circulation loop; the secondary refrigerant circulation loop is connected and exchanges heat with the high-temperature-stage refrigeration circulation loop through a high-temperature-stage refrigeration plate heat exchanger and is connected and exchanges heat with the low-temperature-stage refrigeration circulation loop through a low-temperature-stage refrigeration plate heat exchanger; a refrigerant of the high-temperature-stage refrigeration circulation loop exchanges heat