The invention discloses a defrosting method and a defrosting system for an air conditioner. The defrosting method comprises the following steps of: 10, electrifying the air conditioner for running and heating; 20, judging, namely (A) judging whether the outdoor environmental temperature is smaller than and equal to first preset temperature, (B) judging whether continuous running time of a heating compressor reaches first preset time, and judging whether accumulative running time reaches second preset time, (C) judging whether the continuous running time of the heating compressor reaches third preset time, and making accumulative running time reach fourth preset time, (D) judging whether the temperature of a defrosting sensor Te or Tc is smaller tan and equal to second preset temperature, keeping the temperature for fifth preset time, and making the accumulative running time reach fifth preset time, and (E) judging whether to defrost manually; and 30, determine to perform timing defrost, automatically defrost, manually defrost or not defrost according to a judging result obtained in the step 20. By the scheme, the problems that when the air conditioner is used for heating, a heating effect is influenced due to a relatively long defrosting period, and the air conditioner cannot defrost, or defrosts after a long time because the environmental temperature is relatively low can be solved.