A multi-split air conditioner and control method are provided. The multi-split air conditioner includes an outdoor unit including an oil separator and a four-way valve. The outdoor unit includes a pipeline connecting the oil separator and the four-way valve, and the pipeline includes a first pipeline and a second pipeline arranged in parallel, wherein the first pipeline is provided with a heat storage module and a heat storage module control valve, and the second pipeline is provided with a first control valve. When the outdoor environment temperature satisfies a certain condition, the first pipeline and the second pipeline are controlled, so that at least part of working medium circulates in the first pipeline between the oil separator and the four-way valve through the heat storage module.