The embodiment of the invention provides an outdoor unit, a defrosting control system and a defrosting control method, and relates to the air-conditioning system which can timely and effectively carry out defrosting control on the outdoor unit, and can exit quickly, so that use effect of the outdoor unit is guaranteed. The outdoor unit comprises a heat exchange unit, a four-way valve, a second stop valve, a high-pressure pressure sensor and a processor, wherein the high-pressure pressure sensor is connected to the second end of the four-way valve; the first end of the heat exchange unit is connected to the third end of the four-way valve; the second end of the heat exchange unit is connected to one end of the second stop valve; the heat exchange unit comprises at least two heat exchange modules; each heat exchange module comprises a heat exchange assembly, an electronic expansion valve and a defrosting temperature sensor; the first end of each heat exchanger assembly is connected to the third end of the four-way valve; the first end of each electronic expansion valve is connected to the second end of each heat exchange assembly; the second end of each electronic expansion valve is connected to one end of the second stop valve; and the defrosting temperature sensor is placed in one branch of each heat exchanger assembly.