The present invention provides a defrosting control device, control method and air-conditioning system for an air-conditioning system, wherein the air-conditioning system includes a compressor (1), an indoor heat exchanger (2), an outdoor heat exchanger (3) and a throttling device ( 4), wherein, the control device includes: a judging unit for judging whether the outdoor heat exchanger (3) has reached the level where defrosting is required; an execution unit for controlling the throttling device according to the judgment result of the judging unit Action to defrost when needed. The present invention uses the executive unit to adjust the throttling device to control the defrosting, cancels the original control through the four-way valve reversing, and can effectively prevent the indoor heat exchanger from evaporating and absorbing heat during defrosting, causing indoor The ambient temperature is lowered, which increases the indoor ambient temperature, avoids slow temperature rise, reduces energy loss, reduces noise and abnormal noise, reduces power consumption, improves operational reliability, and prevents problems such as compressor oil shortage and poor lubrication.