Ultralow-temperature heat pump unit and defrosting control method

The invention relates to an ultralow-temperature heat pump unit and a defrosting control method, and the defrosting control system of the ultralow-temperature heat pump unit comprises a first control branch which is connected with a supercooling outlet of a plate heat exchanger and an enthalpy-increasing electronic expansion valve; the defrosting branch is arranged at the bottom of the finned heat exchanger and used for defrosting the bottom of the finned heat exchanger; and the second control branch is connected with the first control branch and the defrosting branch and used for controlling the defrosting branch to defrost the finned heat exchanger. The last section of branch at the bottom of the fin type heat exchanger serves as the defrosting branch, through the first control branch, the second control branch and the defrosting branch, medium-temperature and medium-pressure refrigerants firstly pass through the defrosting branch at the bottom of the heat exchanger to defrost the fin type heat exchanger an