The invention discloses a heat pump unit, a multi-system heat pump unit and a defrosting control method and device of the heat pump unit. The defrosting control method of the heat pump unit comprisesthe steps that when scheduled time is reached after starting or defrosting, a group of heat transfer temperature difference is recorded every other first time at intervals; when current heat transfertemperature is judged to be smaller than a preset second temperature threshold value and the heat transfer temperature difference corresponding to the current heat transfer temperature is greater thana preset temperature threshold value, a group of heat transfer temperature difference recorded every other second time at intervals is updated; and in the recorded heat transfer temperature difference after detection and updating, when continuous multiple heat transfer temperature difference is greater than the temperature threshold value and the accumulative heating duration of the heat pump unit is greater than the smallest heating duration, the heat pump unit is controlled to enter the defrosting. According to the heat pump unit, the multi-system heat pump unit and the defrosting control method and device of the heat pump unit, the actual heat transfer condition of the heat pump unit is known by adopting the rate of change of pipe coil temperature and environmental temperature, then, accurate defrosting is carried out according to the actual heat transfer condition, and the problems that defrosting is not clean and inaccurate and the unit capacity is affected caused by a previous defrosting mode are effectively solved.