The invention discloses an air conditioner and a controlling method, and belongs to the technical field of air conditioners. The air conditioner is provided with a variable-capacity compressor; an operation mode of the variable-capacity compressor comprises a two-stage mode and a double-cylinder mode; an indoor machine of the air conditioner at least comprises two heat exchange units. The controlling method comprises the steps: acquiring indoor temperature and outdoor temperature; controlling the air conditioner to operate in the double-cylinder mode; and when the outdoor temperature reaches a frost-condensing condition, a first heat exchange unit is controlled to operate in a heating mode, and a second heat exchange unit operates in a defrosting mode. According to the controlling method, under the conditions that the indoor temperature in winter is relatively low and the air conditioner is easily to condense frost, the variable-capacity compressor of the air conditioner is controlled to operate in the double-cylinder mode; and according to temperature difference, a heat exchange unit is controlled to carrying out frosting treatment of an outdoor machine of the air conditioner, cooling medium output by the variable-capacity compressor can meet the requirement on cooling medium capacity during heating and defrosting of the heat exchange units of the air conditioner, and the air conditioner can run in an optimal energy efficiency state.